Category Archives: UNMAP

Mind the UNMAP Impact On Performance In Certain Scenarios

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The Problem

Recently we’ve been trouble shooting some weird SQL Server to file backup issues. They started failing on the clock at 06:00 AM. We checked the NICs, the switches, the drivers, the LUNs, HBAs, … but it was all well. We considered over stressed buffers as the root cause or spanning tree issues but the clock steadiness of it all was weird. We tried playing with some time out parameters but with little to no avail. Until the moment it hit me, the file deletions that clean up the old backups!We had UNMAP enabled recently on the SAN.

Take a look at the screenshot below an note the deletion times underlined in red. That’s with UNMAP enabled. Above is with UNMAP disabled. The Backup jobs failed waiting for the deletion process.

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This is a no issues if your backup target is running something prior to Windows Server 2012. if not, UNMAP is disabled by default. I know about the potential performance impact of UNMAP when deleting or more larger files due to the space reclamation kicking in. This is described here Plan and Deploy Thin Provisioning under the heading “Consider space reclamation and potential performance impact”. But as I’m quite used to talking about many, many terabytes of data I kind of forget to think of 500 to 600GB of files as “big” Embarrassed smile. But it seemed to a suspect so we tested certain scenarios and bingo!

Solutions

  1. Disable the file-delete notification that triggers real-time space reclamation. Find the following value HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlFileSystemDisableDeleteNotification and set it to 1.

    Note that: This setting is host wide, so for all LUNs. Perhaps that server has many other roles or needs to server that could benefit from UNMAP. If not this is not an issue.  It is however very efficient in avoiding issues. You can still use the Defragment and Optimize Drives tool to perform space reclamation on-demand or on a scheduled basis.

  2. Create LUNs that will have high deltas in a short time frame as fully provisioned LUNs (aka thick LUNs). As you do this per LUN and not on the host it allows for more fine grained actions than disabling UNMAP.  It makes no sense to have UNMAP do it’s work to reclaim the free space that deleting data created when you’ll just be filling up that space again in the next 24 hours in an endless cycle. Backup targets are a perfect example of this. This avoid the entire UNMAP cycle and you won’t mind as it doesn’t make much sense and fixes you issue. The drawback is you can’t do this for an existing volumes. So it has some overhead & downtime involved depending on the SAN solution you use. It also means that you have to convince you storage admins to give you fully provisioned LUNs, which might or might not be easy depending on how things are organized.

Conclusion

UNMAP has many benefits both in the physical and virtual layer. As with all technologies you have to understand its capabilities, requirements, benefits and draw backs. Without this you might run into trouble.

Join me for aTechNet Live Meeting: Hyper-V Storage Efficiencies & Optimizations in Windows Server 2012 R2

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So you have been  playing with or down right seriously testing Windows Server 2012 and perhaps even Windows Server 2012 R2. That’s great. Many of you might have it running in production or are working on that. That’s even better.

Windows Server 2012 has brought us unseen capabilities & performance enhancements that make it a future proof fundament for many versions to come and it is ready for the ever accelerating pace of hardware improvements. R2 has fine tuned some points and added improvements that are stepping stones to better today and even greater in vNext. I’d like to invite you to a free TechNet Live Meeting on Hyper-V Storage Efficiencies & Optimizations in Windows Server 2012 R2 and look at some of these capabilities with me.

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As a virtualization guy two subjects are very dear to me and that is networking & storage, and this event is about a subset of the storage improvements. You might have heard about ODX and UNMAP but you have not had the change to play with it. You have read about the tremendous scalability of the IOPS in a VM and about large sector support for the next generation of hard disks drives. Well some of these we’ll demonstrate (ODX, UNMAP, Dynamically expanding VHDX performance) if the demo gods are with us. Others we’ll discuss so you’ll know where this comes into play and how you’ll benefit from them even without realizing you do. So without further delay register for the free TechNet Live Event here.

Windows Server 2012 R2 Unmap, ODX On A Dell Compellent SAN Demo

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UNMAP & ODX Video

Some things are easier to show using a video so have a look at a video on UNMAP/ODX used with Windows Server 2012 R2 and Compellent SAN:

You can also go directly to the Vimeo page by clicking on the below screen shotimage

We start out with a 10.5TB large thinly provisioned LUN that has about 203GB of space in use on the SAN. So the LUN on the SAN might be 10.5TB and windows sees a volume that is 10.5TB only the effective data stored consumes storage space on the SAN. That ought to demonstrate the principle of thin provisioning adequately Smile. The nice PowerShell counter is made possible via the Compellent PowerShell Command Set.

We then copy 42GB worth of ISO files inside a Windows Server 2012 virtual machine from a fixed VHD to a dynamically expanding VDHX. Those are nice speeds. And look at how the size of the VHDX file grows on the CSV volume and how the space used on the SAN is growing. That’s because the LUN is thinly provisioned.

Secondly we copy the same ISO files to a fixed size VHDX. Again, some really nice speeds. As the VHDX is fixed in size you do not see it grow. When looking at the little SAN counter however we do see that the thinly provisioned LUN is using more storage capacity.

Once that is done we see that the total space consumed on the SAN for that CSV LUN has risen to 284GB. We then delete the data from both dynamically expanding VHDX and are about to run the Optimize-Volume command when we notice that the SAN has already reclaimed the space. So we don’t run the optimize command. Keep that in mind. By the way, this process is done as part of standard maintenance (defrag) and some NTFS check pointing mechanism that’s run every 5 minutes and sends down the info from the virtual layer to the physical layer to the SAN. During demo’s it’s kind of boring to sit around and wait for it to happen Smile. Just remember that in real life it’s a zero touch feature, you don’t need to baby sit it.

We then also delete the ISO files from the fixed VHDX and run Optimize-Volume G –Retrim and as result you see the space reclaimed on the SAN. As this is a fixed disk the size of the VDHX will not change. But what about the dynamically expanding VHDX? Well you need to shut it down for that. But hey, nothing happens. So we fire it up again and do run Optimize-Volume H –Retrim before shutting it down again and voila.

So what do you need for this?

Rest assured. You don’t need the most high end, most expensive, complex and proprietary SAN hardware to get this done. What you need is good software (firmware) on quality commodity hardware and you’re golden. If any SAN vendor wants to charge you a license fee for ODX/UNMAP just throw them out. If they don’t even offer it walk away from them and just use storage spaces. There are better alternatives than overpriced SANs lacking features.

I’ve found that systems like Equalogic & Compellent are in the sweet point for 90 % of their markets based on price versus capabilities and features.  Let’s look at the a Compellent for example. For all practical intend this SAN runs on commodity hardware. It’s servers & disk bays. SAS to the storage & FC, iSCSI or SMB/NFS for access. With capable hardware the magic is in the software. Make no mistake about it, commodity hardware when done right, is very, very capable. You don’t need a special proprietary hardware & processors unless for some specialized nice markets. And if you think you do, what about buying commodity hardware anyway at 50% of the cost and replacing it with the latest of the greatest commodity hardware after 4 years and still come out on top cost wise whilst beating the crap out of that now 4 year old ASIC and reaping the benefits of a new capabilities the technology evolutions offers? Things move fast and you can’t predict the future anyway.