The Power Question

I've seen a lot of discussion (and I think a bit of a theme at SNW) on power consumption in datacentres. Obviously the subjects of global warming and increased energy prices have put the subject at the centre of focus. But I think when datacentres are being built, there isn't an issue. The problem comes along as the datacentre fills up with equipment. Invariably, new equipment (especially in the storage world) is coming in denser and requiring more power per rack or square metre. So, as equipment is swapped out and replaced, the original calculations done on how much power per square metre is needed are no longer accurate and the balance tips from one of "have we got the space" for new equipment to one of "can we power the new kit up".

I don't see how this problem will be solved as datacentre planners will always cater for the power/cooling of today's products, not the mythical power demands of future products. Datacentres will therefore have a finite life, after which you may as well start again.

Here's a practical example; There is a manufacturer of highly dense storage arrays (that don't need to be powered up all the time) who can't deploy into a number of London datacentres I know because product density would cause the array to fall to the floor. The datacentres were never designed to take products of that weight...

Another Great Idea

I've another great idea for a software product (I have these ideas from time to time, but converting them into reality always proves difficult).

So, museum environments for backups. They're going to be a major headache going forward, even more than they are today as there are more demands on the timely keeping and retrieval of backups. What's needed is a product which can understand legacy backup products and do two things; (a) extract a copy of the backup catalog into a single database, based on a standard schema for backup data and (b) read the content of backup media directly without the need to use the old backup product.

This may seem like backup software companies giving away their IP but I don't think it is. I was in a discussion recently where EMC would not give support on (admittedly very) old versions of Legato, especially with respect to merging catalogs from multiple platforms. This leads to costly and risky options requiring the retention of legacy hardware (subject to failure), legacy software (no longer supported) and legacy media (prone to failure). The lack of a single catalog precludes the ability to easily identify and manage backups and backup images when multiple backup systems exist.

I wonder if any of the vendors out there would be happy to let me have copies and information on the defunct versions of their backup products?

Footnote; I am aware of the backup management products out there like Bocada; to my knowledge none of them actually merge the catalogs into a view at the file level or offer direct media restore.

Storage as a commodity

I just read a comment over at Zerowait regarding Netapp and proprietary hardware. It reminded me of something I was thinking about recently on the commoditisation of storage.

There's nothing worse to my mind than a storage vendor who has no competition. Inevitably in some organisations that situation can exist when a single supplier is chosen to supply (for example) switches, SAN or NAS. The difficulty though is how to avoid that situation. Most vendors would love to lock you into their proprietary tools and relating back to the above article link, Netapp is one I see who try that more than anyone. They have a bewildering array of interlinked product options; once your hooked (especially where you use a feature to retain long term backups via snapshot/vaults) then you're sucked into a dependency on their products which just isn't healthy.

What's the solution? Well, for me I like to commoditise storage functionality. Pick out those features which all vendors support and only use the proprietary features where absolutely necessary. At least then you can maintain multiple vendors all on the hook for your next piece of business.

Of course implementing commoditised storage is more difficult than just picking a few common product features. However as far as your users are concerned, a LUN is a LUN and NAS storage is NAS storage, with a few caveats on things like driver levels for HBAs and so on.

I've previously posted a modular storage comparison sheet. As an example, here are some of the features that almost all support:

• RAID 5 protection
• Consistent LUN size
• dual pathing
• Active/Passive failover
• remote replication
• Fibre Channel presentation
• SNMP alerting
• online code upgrades
• hot swappable components

Before I get lots of comments saying "hold on, not all modular products are the same"; remember I'm not saying that. What I am saying is having a consistent set of requirements allows you to maintain a shortlist of vendors who can all enjoy the healthy competition of bidding for business. So, time to draw up a NAS spreadsheet....

AoE/FCoE/iSCSI

Robin harris discusses AoE from Coraid. I looked at this last year (reminder) as I saw it as a great way to get a FC/iSCSI solution at a low cost. However, before everyone rips out their FC SANs and runs to put an Ethernet solution in place, take one step back and consider the issues. Fibre Channel is successful because it works; because it is reliable. FC switches have features such as non-blocking architecture, QOS, preferred path and so on which help to remove or eliminate throughput or performance issues. Would ATA over Ethernet (or for that matter as it seems to be a topic of the moment) FC over Ethernet provide for that level of switch point to point bandwidth guarantee?

Consider also your monitoring tools. Both Brocade and Cisco offer features to do traffic redirecting (e.g. SPAN ports) to easily analyse SAN traffic without putting TAPs in place. Will AoE and FCoE offer that?

Consider security. Will AoE provide the same level of security as FC?

Without a doubt, you get what you pay for; however you should only pay for what you *need*. If you are running a mission critical application FC is still the best option - interoperability is more widely tested; diagnostic tools are mature; the technology is reliable. I do think there's a place for AoE, iSCSI and FCoE, but use it in the wrong place and what you save in cost, you may pay for later in downtime.

Where are all the simulators

I love the Netapp simulator (well, apart from the annoying issues with creating and deleting disks) and I use it all the time. It is great for testing ideas, testing scripting and generally refreshing knowledge on commands before having to touch real equipment. I use it with VMware (as I have probably mentioned before) and I can knock up a new environment in a few minutes by cloning an existing machine. Netapp have got a huge advantage in offering the tool as it enables customers who can't or won't put in test equipment to do work and protect their production environments.

So, where are all the other simulators? Is it just that I don't know they exist or do most vendors not provide them? For the same reasons as I mentioned above, if there were simulators for EMC DMX, HDS USP, Cisco and Brocade/McDATA switches, then there would be a huge opportunity for people to test and develop scripts, test upgrades and other useful work.

Would anyone else like a simulator? Can the vendors tell me why they don't produce them?

Improving efficiency

Hu posted an interesting view of storage utilisation here. His view is that virtualisation on the USP would improve on the average 30% utilisation. I have to say that I disagree with this. There are lots of reasons why storage remains unused in the enterprise:

1. inefficient administration by Storage Admins, SAs and DBAs (lost/orphan disks etc)
2. deliberate overallocation by SAs and DBAs (in an attempt to manage change control versus growth)
3. in process tasks (frame/host migrations, delayed decommissions)
4. data distribution for performance management
5. Storage Admin "buffers" to manage growth.

Many of these issues are process driven rather than due to the inadequacies of technology. Now, "true" virtualisation may address some of the problems listed above, especially those technologies which provide for thin provisioning or other overallocation methods. There are plenty of technologies on the market already offering this style of allocation however there are obviously shortcomings with the technology that could hold it back; most notably are performance and reporting.

Performance is seen as a problem due to the overhead of having to work out where blocks have been virtualised to. In addition, I/O must be written to the virtualisation device and rewritten to the physical storage creating a "dual" write scenario. Data distributed across multiple devices may not provide the same performance profile and lead to uneven response times. In fact this scenario already exists in existing enterprise storage. Data is written to cache and destaged at a later time; arrays may contain more than one type of disk size and type; data is mapped across physical disks in a RAID configuration.

Reporting is an issue as most people like to know where their data resides. There are good reasons for this; if hardware fails or if any disaster scenario is invoked then it is important to know what data is pinned where; is it still left in cache? If a RAID group fails, what data did it contain and from which volumes? In addition, overallocation creates its own problems. Predicting how virtual volumes will increase their storage usage is tricky and you most certainly don't want to get caught out refusing I/O write requests for production disks. This issue was very obvious with the early implementation of thin provisioning on Iceberg (a StorageTek storage subsystem) where reporting failed to cope with volumes that had been copied with snapshot.

Now, if HDS were to add thin provisioning to the USP, how good would that be....

Distributed Backup

Following on from a previous post on RAID and backup, I've been doing some more thinking on how to back up consumer data from a PC workstation. I reckon I've got about over 200GB of data on my server, which previously was on my main workstation. I dabbled with the Linksys NSLU2 however I hated it; I was really nervous (a) about the fact I would lose access to the device (b) it couldn't cope with the volume of files and seemed to lose track of what I had allocated and (c) how I would recover the data from the USB drives I used if the device eventually packed up. In fact, I got rid of the NSLU2 when it did lose track of my data. I was lucky to find a Windows read-only driver capable of reading the NSLU2 format and I got my data back.

Getting back to the question in hand, how would I back up 200GB? I guess I could fork out a few grand for an LTO drive and tapes, but that's not cost effective. I could do disk to disk copy (which I do) but D2D isn't as portable as tape and much more expensive if I intend to maintain multiple copies. I should mention that I've automatically discounted DVD and HD-DVD/Blu-Ray due to lack of capacity and cost (the same applies to the latest optical drives too).

I could use one of the many network backup services on offer. About 10 years ago, I looked at the feasibility of setting up one of these services for the storage company I worked for at the time. It was almost feasible; Freeserve was doing "free" dial-up internet (you paid for just the cost of the calls) and companies such as Energis were selling virtual dial-up modems on very good terms. However the backup model failed as the cost to the customer just didn't stack up due to the length of time to copy files out to the backup service.

I think network backup services *could* be the best answer to safeguarding your PC/workstation data. However the existing services have issues for me; basically I don't trust someone else with my data, which could include bank details, confidential letters and files. Even if I can encrypt my data as it is transmitted to the network backup service, they still have *all* of my data and with enough compute power could crack my encryption key.

If anyone has examples of services which could provide 100% security, I'd be interested to know.

So, here's my idea. a distributed backup service. We all have plenty of free space on those 500GB drives we've installed. Why not distribute your backups amongst other users in a peer to peer fashion? There are two main drawbacks to my mind. First, how can I guarantee my data will always be available for me to access (PCs may be powered off) and secondly, how can I guarantee security?

Existing P2P services work by finding as many servers/PCs as possible which hold the data you want to download. Many may not be online; many may be online and running slow. By locating multiple copies of the required data, then hopefully one or more will be online and available for download.

The same can be applied to backups; split files up and distribute the fragments to P2P servers and index where they are. The fragments would need to be encrypted in some way to guarantee anonymity but make them common to files on both your machine and others. You then maintain an index to rebuild the backup data; all that then needs to be backed up is the index which could easily fit onto something like a CD-ROM. All data could then be recovered using just a small CD index, which could be recreated from anywhere.

There are a lot of unanswered issues; how would data be encrypted; how would the fragments be created and "de-duplicated", how would fragments be distributed across the P2P members to ensure availability? How would the fragments be created to prevent the actual original files from being discovered?

Still, it's only a concept at this stage. But using the internet and all that unused disk space out there could prove a winner.

Giving RAID the thumbs up

Just read Robin Harris' post at his new blog location; http://blogs.zdnet.com/storage/?p=116 and his comment on another blog discussing RAID. He quotes a VAR who has tracked disk failures and thinks RAID is an expensive luxury for desktops.

It's interesting to see the failure rates quoted, anywhere from 1-3%, which on the face of it, seems low. However when its *your* disk that has failed and the data is irretrievable, there's cold comfort to be had in failure rate statistics. I run RAID1 on my server; I have two 500GB SATA drives. Backing up that volume of data on a regular basis is a nightware without investing in a very expensive backup solution like LTO and it is a real disappointment to see tape hasn't kept pace with disk in terms of the capacity/cost ratio.

So, I'm sticking with RAID. I augment it with disk-to-disk backups because, yes, you do have to cater for the d'oh factor of user errors or even dodgy software which corrupts files too, but RAID works for me and that's all I need to worry about.

Uh Oh Domino

It seems that the world is moving to Exchange for email messaging. Unfortunately there are some of us still using Lotus Notes/Domino.

As a messaging product, it seems to me to be reasonably efficient; our Domino servers can support upwards of a thousand users, perhaps 1-2TB of Notes mailboxes. Domino stores the mailboxes as individual files with the .nsf extension. Each of these is opened and held by the nserver.exe task. When using Netbackup with the Notes/Domino agent, the Netbackup client backs up all nsf files on a full backup and the transaction logs and changed nsf files (i.e. those with a new DBID) for an incremental backup. This creates a significant amount of hassle when it comes to performing restores.

A restore is either the full nsf file from the last full backup, or the nsf file plus transaction logs, which are then applied to the nsf file to bring the mailbox up to date. This process is incredibly inefficient because (a) transaction logs contain data for all users and must be scanned for the records relating to the restoring mailbox (b) the transaction logs need to be restored to a temporary file area, which could be considerable (c) the restored logs are disregarded after the restore has completed and so have to be restored again for the next mailbox restore.

So, I’ve been looking at ways to bin Netbackup and improve the backup/restore process. As servers are being rebuilt on Windows 2003 Server, I’ve been looking at VSS (Volume Shadowcopy Services). This is a Windows feature which permits snapshots of file systems to be taken in co-operation with applications and underlying storage. In this instance there isn’t a Lotus Domino provider, so any snapshots taken are dirty (however I did find the dbcache flush command which flushes updates and releases all nsf files). Netapp used to have a product called SnapManager for Lotus Domino which enabled Netapp snapshots of mailboxes using the Domino Backup API. The product has been phased out, as tests performed by Netapp show that dirty snapshots with the security of logs can be used to restore mailboxes successfully. IBM provide trial versions of Domino, so, I’ve downloaded and installed Domino onto one of my test servers under VMware and run the load simulator while taking snapshots with VSS. I’ve also successfully restored a mailbox from a snapshot so there’s no doubting the process works. However my simple task isn’t one of scale. Typical mailboxes are up to 1GB in size and there could be hundreds of active users on a system at any one time. My concern is whether VSS can manage to take snapshots with this level of activity (and not impact the O/S) but also whether the snapshots will be clean or what level of corruption we can expect.

The only way to test this is to implement on a full scale Domino environment and probably with live users. That’s where things could get interesting!

WWN Decoder

As pointed out by Richard, my WWN decoder stopped working when I redid my website. Here's a new link; http://www.brookend.com/html/resources/wwndecoder.asp.

Developing a Tiering Strategy

Implementing a storage tiering strategy is a big thing these days. Everyone should do it. If you don't then you're not a "proper" storage administrator. Being serious and moving away from the hype for a second, there is a lot of sense in implementing tiering. It comes down to 1 thing - cost. If disk and tape storage was free, we'd place all our data on the fastest media. Unfortunately storage isn't free and therefore matching data value to storage tiers is an effective way of saving money.

Choosing the Metrics

In order to create tiers it's necessary to set the metrics that define different tiers of storage. There are many to choose from:

• Response time
• Throughput
• Availability (e.g. 5 9's)
• Disk Geometry (73/146/300/500GB)
• Disk interconnection (SATA/FC/SCSI)
• Usage profile (Serial/Random)
• Access Profile (24x7, infrequent)
• Data value
• Array Type (modular/enteprise)
• Protection (RAID levels)

There are easily more, but these give you a flavour of what could be selected. In reality, to determine the metrics to use, you really need to look at what would act as a differentiator in your environment. For example, would it be really necessary to use 15K speed drives rather than 10K? Is availability important - should RAID6 be considered over RAID5? Is there data in the organisation that would exist happily on SATA drives rather than fibre channel? Choosing the metrics is a difficult call to make as it relies on knowing your environment to a high degree.

There are also a number of other options to consider. Tiers may be used to differentiate functionality, for example tiers could be used to specify whether remote replication or point-in-time copies are permitted.

Is It Worth It?

Once you've outlined the tiers to implement, you have to ask a simple question - will people use the storage tiers you've chosen? Tiering only works if you can retain a high usage percentage of the storage you deploy - it's no use deploying 20TB of one tier of storage and only using 10% of it. This is a key factor. There will be a minimum footprint and capacity which must be purchased for each tier and unless you can guarantee that storage will be used, any saving from tiering may be negated by unused resources. Narrow your tiering choices down to those you think are actually practical to implement.

Making the Move

So, the tiers are set, storage has been evaluated and migration targets have been identified. How do you make it worthwhile for your customers to migrate? Again, things come back to cost. Tiers of storage will attract differing costs for the customer and calculating and identifying the cost savings will provide a justification for investing in the migration. In addition, tiers can be introduced as part of a standard technology refresh - a process that regularly happens anyway.

Gotcha!

There are always going to be pitfalls with implementing tiering:

1. Don't get left with unusable resources. It may be appealing to identify lots of storage which can be pushed to a lower tier. However, if the existing tier of storage is not end-of-life or unless you have customers for it, you could end up with a lot of high tier unused storage which reflects badly on your efficiency targets. Make sure new storage brought in for tiering doesn't impact your overall storage usage efficiency.
2. Avoid implementing technology specific tiers which may change over time. One example; it is popular to choose to tier by drive size on the assumption that higher capacity drives offer a lower performance and therefore are matched to a lower tier. But what happens when the predominant drive type changes or you buy a new array in which the larger drives perform equally well compared to an older array? How should those tiers be classified?
3. Be careful when choosing absolute parameters for tiers. For example, it is tempting to quote response time characteristics in tiers. However, no subsystem can guarantee consistent response times. It may be more appropriate to set confidence limits, such as offering "<>

Iterative Process

Developing a tiering strategy is an iterative process which will constantly be refined over time. There's no doubt, that implemented correctly, it will save money. Just don't implement it and forget about it.

Standards for Measuring Performance

I had a bit of banter following a post I made on ITToolbox earlier in the week. The question was posed as to why major disk array vendors (other than Netapp) don't produce performance statistics. There are benchmarks; the Standard Performance Evaluation Corporation and Storage Performance Council for instance. However SPEC only covers NAS and neither HDS and EMC sign up for SPC.

Producing a consistent testing standard is extremely difficult. Each storage array vendor has designed their hardware to have unique selling points and more importantly, each vendor can't make their systems too close to their rivals otherwise there'd be more money made by lawyers than the vendors themselves.

Let's pick a few examples. In terms of architecture, DMX (EMC) and USP (HDS) have a similar design. They both have front-end channel adaptor cards, central cache and back-end loops on which (probably identical) hard disk drives are connected. However the way in which the physical storage is carved up is totally different.

HDS uses the concept of an array or parity group; a 6+2 RAID group has 8 disks of the same size which can be carved up into logical units (LUNs). The address at which these LUNs are mapped is up to the user, but typically LUNs will be dispersed across multiple array groups to ensure that consecutive LUNs are not mapped to the same physical disks. This process ensures that data is hitting as many spindles as possible.

EMC chooses another method. Each physical drive is divided into hypers, or logical slices. These are then recombined to make LUNs. RAID 1 LUNs have 2 hypers, RAID5 LUNs have 4 hypers. Each hyper is taken from a different back-end drive loop to improve performance and resiliency.

Now, the comments earlier in the week referred to Netapp. Their architecture comes from a NAS file serving design, which uses RAID 4 and has all the operations for RAID calculations handled in memory. LUNs are carved out of RAID "volumes" or aggregates.

So what is a fair configuration to use? Should it be the same number of spindles? How much cache? How many backend loops and how many front-end ports? Each vendor can make their equipment run at peak performance but choosing a "standard" configuration which sets a level playing field for all vendors is near impossible. Not only that, some manufacturers may claim their equipment scales better with more servers. How would that be included and tested for?

Perhaps rather than doing direct comparisons, vendors should submit standard style configurations, based on a GB capacity on preset LUN sizes against which testing is performed using common I/O profiles. This, with list price would let us make up our own minds.

Either way, any degree of testing won't and shouldn't stop healthy discussion!

Software Shortcomings

I was invited last week to speak at the first (of hopefully) many HDS User Forums in the UK. The subject was Storage Resource Management and I talked on the subject and my thoughts for about 30 minutes. One slide generated the most interest and I've included it here. It shows some of the issues current SRM products have and which are not really being addressed by software vendors.

A case in point is the installation of Device Manager I performed this week on a virtual domain. HiCommand Device Manager 5.1 is supported on VMware (2.5) however I couldn't get the software to install at all. I tried the previous version which worked fine, so I was confident the Windows 2003 build was OK. HDS pointed me at a new feature I'd not seen before, Data Execution Prevention which is intended to prevent certain types of virus attacks based on buffer overflows. Whilst this solved the problem, it didn't fill me with a great deal of confidence to think Windows judged HDS's software as a virus. With DEP enabled, the installation got further but still eventually failed. On HDS' advice, re-running the installation again worked.

At the forum, HDS presented their SRM roadmap. If it all comes to fruition then I'll be able to do my provisioning, monitoring and other storage management tasks using my Blackberry whilst sipping Pina Colada's on a Carribbean beach. Back in the real world, my concern is that if the existing tools don't even install cleanly, how am I expected to trust a tool which is moving my disks around dynamically in the background?

It's easy for me to target HDS in this instance but all vendors are equally culpable. I think there's a need for vendors to "walk before they can run". Personally, I'd have more trust in a software tool that was 100% reliable than one which offered me lots of new whizzy features. Vendors, get the basics sorted. Get confidence in your tools and build on that. That way I might get to do provisioning from the beach before I'm too old and grey to enjoy it.

Backing Up Branches

I've spent the last few weeks being heavily involved in a storage performance issue. Unfortunately I can't discuss the details as they're too sensitive (in fact, I can't even mention the TLA vendor in question), however it did make me think more about how we validate a storage architecture design will actually work.

As part of another piece of work I'm doing, I have been looking at getting branch or remote office data into core datacentre infrastructure, the premise being not to write tapes in remote locations and therefore be at risk of compromising personal information.

I've looked at a number of solutions, including the Symantec/Veritas PureDisk option (the flashy flash demo can be found here). What strikes me with this and other solutions is the lack of focus on data restores and meeting RTO and RPOs for applications that may run in branch locations. In fact, I think most products are aimed at the remote office which runs a small Windows implementation and therefore isn't worried too much about recovery time of their data.

The technologies such as PureDisk work by reducing the data flowing into the core datacentres, either by de-duplication, compression or both technologies. This is fine if (a) your data profile meets their view of compression (b) your applications/data produce a small percentage of changed data on a daily basis. Unfortunately for some applications this just doesn't occur. In addition, if you lose the site (or even easier, corrupt or lose the local metadata database) then you're looking at a total restore from core to recover an entire server. These solutions work on the basis that branch offices have low WAN bandwidth and the backup tools utilise this by making a small pipe appear as a much larger pipe through the aforementioned de-dupe and compression. This doesn't help if a full restore needs to be performed.

Some people may say "but you could manage the restore by sending a tape to the site containing the last backup" - however that defeats the original objective of keeping tapes out of the branch.
I'd like to see some real-world examples from Symantec/Veritas showing how long backup/restores would take with their product. Of course they're not going to provide that as it would immediately highlight the issues involved. Perhaps we need an independent backup testing lab to do that comparison for all backup products on the market.

iSCSI Part 3 (or is it part 4)

I've done the last part of my iSCSI evaluation. Previously I looked at the protocol itself and the ways to secure data. Transmission needs to be secured by either IPsec for a shared network or a dedicated network infrastructure. The last part of the jigsaw I checked out was how to validate the client; basic iSCSI authentication uses simply the name of the iSCSI initiator, which is nothing more than a plain-text field.

The standard iSCSI method is to use CHAP. This requires both the client and the target to provide a username and password for login to each other. I tested it out on my Netapp Simulator/Windows environment and of course it works. What I'm not sure about is how effective this is as a security method. It is necessary to retain password information for both client and target and store it elsewhere; there's no 3rd party authentication authority. Perhaps I'm being a little paranoid.

So there it is. I now know iSCSI. I have to say I like it. It's simple. It works. It is easy to implement. Security could be better, and could certainly be made more easy to manage, but perhaps that is related to the two implementations I've used (i.e. Netapp and Windows).

So what are the iSCSI best practices? I'd say:

1. Implement CHAP for client/target security.
2. Implement IPsec to encrypt your iSCSI traffic.
3. Place your devices on a dedicated network.
4. Use dedicated network cards where possible.

I hope to do a practical implementation of iSCSI soon. I can really see it as a practical alternative to fibre channel.

Is ASNP another SNIA?

I received this email this week for the very first ASNP UK Chapter meeting (ASNP is a US based user association for storage people):

<------ START ------>
Dear ASNP Chapter Members of the UK,

Due to low registration numbers, we have decided to cancel the February 23rd ASNP Chapter meeting which was to be sponsored by QLogic.

My apologies for any inconvenience this may cause.

Meanwhile, we are working on a quick survey that we hope you will answer as we’re hoping to find out what type of meetings you’d like to be a part of going forward.

Please keep your eyes open for that survey and let us know!

<----- END----->

Until November I was UK Chair. I found it difficult to work out what ASNP was for; I couldn't see it collectively lobbying storage software/hardware vendors with issues or for new features, in fact all I could see was an organisation which occasionally offered vendor sponsored "training" or information sessions. I even attended the very first ASNP Summit in Long Beach in June 2004; despite registering, when I got there, no-one could find my registration details; everything was extremely US-focused.

If an organisation exists to further the interests of its members, then there should be a clear statement of what those aims are; otherwise why bother to exist at all?

Will ASNP continue? Probably. I notice though that they still don't charge for membership. Unless they start having more to offer, I can't see how they ever can.

Long term data retention

I spent some time earlier this week talking to COPAN. They produce high density storage systems, but not the sort of arrays you'd use for traditional data storage. Their product is pitched at the long term persistent storage market.

I'm sure you can read the website if you're interested however, I hadn't really thought through what this kind of technology could deliver. There are some fundamental issues the storage of "persistent" data needs to be sorted. For instance; how do you validate the data on your disk will still be there when you come to read it 12 months later? (disk Aerobics is the answer apparently; regular validation of disk content).

So, the target market for COPAN is long term data archive. They want you to keep your data on disk. Personally, I think if the price is right, then backup data on disk is a sensible proposition. Today's network connectivity and encryption technology means data doesn't need to be physically moved any longer. In fact, I'd suggest that removing the need to physically move data is the way forward. Disk-based data is inherently more reliable and accessible. COPAN (and others) have plenty of features that can make disk-based backup work.

Don't move the media. Just move the data. Sounds like a good strapline.

Snow!

Totally off-subject but it snowed today in the UK. Most people hate it, I love it. We had this snowman built before breakfast.....

Write Acceleration

Reading Sangod's recent post on write acceleration, I couldn't help writing a response, as I've been looking at this whole subject recently.

First of all, I don't disagree with the concept of synchronous. Yes, the I/O must be confirmed (key word there) at the remote and local site before the host is given acknowledgement of the I/O being complete. Typically, enterprise arrays will cache a user I/O, issue a write to the remote array (which will also be cached), acknowledge the I/O to the host and destage at some later stage.

Sangod mentioned two techniques; cranking up buffer credits and acknowledgement spoofing. Buffer credits are the standard way in which the fibre channel protocol manages flow control. As each FC device passes data (for example an HBA to a switch) then the device can keep sending packets of data whilst the receiving device issues the R_RDY signal back. Buffer credits are essential as FC packets take a finite amount of time to travel a fibre optic cable. The longer the distance between devices, then the more packets which must be "on the line" in order to fully utilise the link. The rule of thumb is 1 buffer credit for each km of distance on a 2Gb/s connection. So if you don't have enough buffer credits, then you don't make best use of the bandwidth you have available. Having lots of data in transit does not compromise integrity as nothing has been confirmed to the originating device.

Moving on, there are devices which can perform write acceleration by reducing the SCSI overhead by acknowledgement spoofing. I've taken the liberty of loaning a couple of graphics from Cisco to explain how a SCSI transaction works.

When a SCSI initiator (source device) starts a write command, it issues a FCP_CMND_WRT to the target device. This is confirmed by the target with an FCP_XFER_RDY. The initiator then issues data transfer (FCP_DATA) repeatedly until all the data is transferred. The target confirms successful receipt of all of the data with FCP_RSP "Status Good". The initial preamble as the write request is started can be reduced by the switch connected to the initiator issuing an immediate FCP_XFER_RDY, allowing the initiator to immediately start sending data. The data transfer and the FCP_CMND_WRT operate in parallel, saving the time of this part of the exchange. No integrity is risked as nothing is confirmed by the target until all data is received.

I see no issue with this kind of spoofing as the initiator is not being told about the completion of the I/O before it has actually happened. What I do see as a concern is that the target may not be able to accept the data and therefore if the write fails, the source needs to be able to roll back if that happens.

In terms of getting best performance, multiple replication groups (e.g. multiple RA groups) would make best use of any WA technology. Cisco publish stats which show that. So WA could be good and safe.

Storage protocols for VMware

I've been doing more VMware work recently. The deployment I'm working on is using SAN presented disk. The storage started as 50GB LUNs, quickly grew to 100GB and now we're deploying on 200GB LUNs, using VMFS and placing multiple VM guests on each meta volume.

Now, this presents a number of problems. Firstly, it was clear the LUN sizes weren't big enough in the first place. Second, migrating guests to larger LUNs had to be an offline process; present the new LUNs, shutdown the guest, clone the guest, restart the guest, blow the old guest away. A time intensive process, especially if it has to be repeated regularly.

Using FC presented LUNs/metas also presents another problem; if we choose to use remote replication (TrueCopy/SRDF) to provide DR failover then all the VM guests on a meta have to go on that failover too. This may not (almost certainly not!) be practical.

Add in the issue with lack of true active/active multipathing and restrictions on the number of LUNs presentable to an ESX server and FC LUNs don't seem that compelling.

The options are to consider iSCSI or store data on CIFS/NFS. I'm not keen on the CIFS/NFS option, iSCSI seems more attractive. It pushes the storage management away from the ESX Server and onto the VM guest; security is managed at the array level, rather than within ESX. Personally I think this is preferable let ESX (system) administrators do their job etc etc. One last benefit; I can present as many iSCSI LUNs as I like of whatever size. It means I can also stripe multiple LUNs; something I'm unlikely to do on VMFS presented devices.

Therefore I think iSCSI could be a great option. Then I thought of one curve ball; what if I could do thin provisioning on FC? Here's the benefit. Imagine creating 20 VM guests on a server, all running Win2K3. Standard deployment is 10GB for the root/boot disk but I'm only actually using about 5. The remainder is left to allow for maintenance/patching/temporary space (we don't want to have to rebuild servers) - applications and data go on separate volumes. I'll use a 200GB meta. Unfortunately it's 50% wasted. But bring in thin provisioning and I can allocate 10GB drives with impunity. I can allocate 20 or 30 or 40! FC is back on the menu. Incidentally, I'm more than aware that iSCSI devices can already be presented thin provisioned.

Lots of people tell me, why bother with thin provisioning. I think in VMware I've found a perfect usage.