VMdamentals

Posts Tagged ‘ESX’

A lot of people have discovered yet another excuse why their environment is not quite performing as it should: misalignment. Ever since a VMware document stated misalignment could potentially cost you up to 60% of performance, it has become an excuse. When looking closer, the impact is often nearly negligible, but sometimes substantial. Why is this?

Introduction

It is more and more seen in VMware environments today. “You should have aligned the partition. No wonder performance is bad”. But what is misalignment exactly, and is it really that devastating in a normal environment? The basic understanding of misalignment is rather simple. In RAID arrays, there is a certain segment size (see Throughput part 2: RAID types and segment sizes). This means data is striped across all members of a raid volume (a set of disks strung together to perform as one big unity). Especially when performing random I/O (and most VMware environments do), you want only a single disk to have to perform a track seek in order to get a block of data. So if your segment size on disk is 64KB, and you read a block of 64KB, only one disk has to seek for the data. That is, IF you aligned your data. If somewhere in between the data is not aligned with the segments on disk, you’d possibly have to read two segments, because each segment carries part of the block to be read (or written for that matter). Exactly that is called misalignment.

In most VMware environments, there are two “layers” between your VM data and the segments on disk: the VMFS and the file system inside your virtual disk. Since ESX 3.x, VMware delivers 64KB alignment of the VMFS. As soon as the blocks vSphere is accessing get bigger than 64KB, you could call it sequential access, where alignment does not help anymore. So basically the start of a VMFS block of 64KB, is always aligned to a 64KB segment on the disks laying underneath. For those who might wonder: VMFS block sizes (1MB … 8MB) are not related to the I/O sizes used on disk; VMFS is able to perform I/O on subsets of these blocks.

The second “layer” is more problematic: The guest file system. Especially NTFS under Windows 2003 server (or earlier) or desktop releases prior to Windows 7, NTFS will by default misalign. I have never understood why, but a default NTFS will align itself to 32256 bytes, or 63 sectors. After that the actual data starts. Getting NTFS aligned is simple: just create a gap after sector 63 right up to sector 128 (or any power of two above for that matter). This is easily done for new virtual disks, but not so easy for existing ones (especially system disks).

Misalignment shown graphically

A lot of people find misalignment hard to understand. A picture says a thousand words, so in order to keep this blog post somewhat shorter: pictures!

In figure 1, both VMFS and NTFS have been properly aligned, including some alignment space. In effect, for every block accessed from or to the NTFS file system, only one block on the underlying storage is touched. Thumbs up!

A misalignment of both VMFS and NTFS is depicted in figure 2. This is a really undesirable situation. As you can see, the access of an NTFS block will require one VMFS segment to be read, sometimes even two (due to NTFS misalignment). But since VMFS is misaligned to the disk segments, every 64KB VMFS block in this example will require the access to two segments on disk. This can and will hurt performance. Luckily, VMware spotted this problem relatively early, and from ESX 3.0 and up VMFS alignment is automagically if you format the VMFS from the VI client.

Figure 3 shows the situation I mostly see in the field. VMFS is aligned (because VMFSses formatted in the VI client align automagically to a 64KB boundary). NTFS is misaligned in this example. I see this all the time in Windows 2003 / Windows XP VMs. As you can see in this example, most blocks touch only a single segment on the physical disk. Some NTFS blocks “fall over the edge” of a 64K segment on disk. Any action performed on those NTFS blocks will result in the reading or writing of TWO segments on the underlying disks. This is the performance impact right there.

You can probably see where this is going: If your segment size on your storage is way bigger than the block size of your VM file system, impact is not too much of a problem. In the example in figure 2, two NTFS blocks out of every 64 blocks will be impacted by this, and only for random access (in sequential accesses your storage cache will fix your problem since both segments on disk will be read anyway). This is an impact of 1/32th, or 3,1%. You could possibly live with that…

Now let’s up the stakes. What if your storage array used a really small segment size on physical disk, let’s say 4KB? Take a look at figure 4:

vSphere will generate I/O blocks which get sized to the highest effectiveness. For example, if you have a database which uses 4KB blocks, and performs 100% random I/O, you get a situation like in figure 4. Every time you access a 4KB block, VMFS interprets this to a 4KB I/O action to your array. Because the NTFS / database blocks are misaligned, EACH access to a 4K block ends up on TWO disk segments. This impacts performance dramatically (up to 50% if all I/O sizes are 4KB). A similar situation occurs when your database application would use 8KB blocks; in that case for every I/O three segments on disk would be accessed instead of two, impacting performance of the disk set by 33% (if all I/O sizes are 8KB).

Why ever use a small segment size?

When you look at an EMC SAN (Clariion), the segment size is fixed at 64KB . When you look at a NetApp, segment size is fixed at 4KB. It would be pretty safe to say, that the impact of misalignment will hit harder on a NetApp than on an EMC box. That is probably why NetApp hammers so hard on alignment; in a NetApp environment it really does matter, in an EMC environment, a little less.

Looking at it the other way round: Why would you ever use such a small segment size? Why not use a segment size of for example 256KB, and feast on having only 1/128th or 0,78% impact when not aligning? Well, using a large segment size appears to be the solution to misalignment. And in a way, it is. But do not forget: Every time you need to access 4KB of data, 256KB is accessed on disk. So both yes AND no, a large segment size makes alignment almost a waste of time, but it introduces other problems.

Somewhere, the “perfect segmentsize” should exist. Best of both worlds… The problem is… This perfect segment size will vary with the type of load you feed to your SAN. EMC is sure about their 64KB (since it cannot be altered), NetApp seems sure about 4KB, because of the very same reason. The el-cheapo parallel-SCSI array (yes parallel SCSI indeed and vmotion works- but that is another story) I use for my home lab does a more generic job: For each RAID volume, I am allowed to choose my segment size (called a stripe size there). Now THAT gives room for tuning! And room for failure in tuning it at the same time…

Dedup and misalignment

Now that deduplication is the new hype, misalignment is said to impact dedup effectiveness. The answer to this, as usual, is…. It depends. If you take two misaligned windows 2003 servers from a template, you’d deduplicate them very effectively since they are very alike. If you were to align one of them (leaving the second one misaligned), dedup would possibly not find a single block in common. Makes sense right? Your alignment shifted all data within the VMDK, differentiating all blocks in effect. If I now align the second VM as well (using the same alignment boundary), dedup would once again be able to work effectively.

So the final answer should be: If dedup is to be effective, either align ALL VMs, or align NONE.

How to get rid of misalignment

Let’s say you’ve found that your VMs are misaligned. If things are really bad they are situated on a RAID volume with a very small segment size. Alignment could save the day. So how do you go about it? Several solutions I’ve come across:

Manual Alignment is perfect for data drives. The idea is that you add a second data drive, create an aligned partition there using diskpart:

after that, stop whatever service is using your datadrive, copy all data, change the drive letters so your new aligned disk matches the old data drive, restart your services, remove the original data disk from the VM. This works great for SQL, Exchange, fileservers etc. The big downside: You cannot align system disks using diskpart (not even from another VM; diskpart’s create partition is destructive).

GParted is a utility that is said to align your partition if you resize the partition using this tool. Never looked into it, but it’s worth checking it out.

Vizioncore’s vOptimizer is a very nice tool that performs alignment for you. Basically it shuts the VM in question, and starts to move every block inside your VMDK(s). You end up with all disks aligned. The VM is then restarted and an NTFS disk check is forced. After that you’re good to go. It served me well on some occasions! You even get two alignments for free if you decide to give their product a spin.

NetApp customers get an alignment tool for free: mbralign. I never used this tool, but apparently it does about the same job as vOptimzer. It shuts your VM, aligns the disks, reboots your VM. It only works on ESX though (installs software in the Service Console).

If you cannot live with the downtime, but need to align anyway, you could consider to look at Platespin products. They can perform a “hot” V2V and align in the process. When data moving is complete, they fail over from the original VM to the newly V2Ved VM, syncing the final changes on the destination disk(s). You end up with an aligned copy of your VM with minimal downtime.

How to prevent misalignment in the first place

Misalignment is often seen, but not necessary at all if you think about it before you start: A lot of people create templates. Not too many align their templates… But you could! If you have a (misaligned or not) VM laying around, you could add an empty system disk of the template-to-be to it, and format the partition aligned from that “helper” VM (see the diskpart description above). Then detach the system disk from the helper VM again, and proceed to install Windows on the (now aligned) disk. Choose not to change anything to the partitioning and you are good to go. Bootable XP CD’s can also do the same trick here.

Now your template is aligned. The upshot: Any VM deployed from this template is too!

There is an easy way to check under windows if your disks are aligned. Simply run the msinfo32.exe from windows, expand components, storage, disks. Find the item “Partition Starting Offset”. If it reads 32.256, you’re out of luck: your partition is misaligned. If it reads 65536, you have a 64K aligned partition. If the value reads 1.048.576, the partition is aligned on a 1MB boundary (Windows 2008 / Windows 7 default).

Conclusion

Is alignment important? Well, it depends. It particularly depends on the segment size used within your storage array. The smaller the segment size, the more impact you have. Bottom line though: Alignment always helps! Get off to a good start and perform alignment right from the beginning and you’ll profit ever after. If you didn’t go off to a perfect start, consider aligning your VMs afterwards. Start with the heavy random I/O data disks for sure, but I would recommend to have the system disks aligned as well, using one of the described tools.

It is certainly not unheared of – “When I delete a snapshot from a VM, the thing totally freezes!“. The strange thing is, some customers have these issues, others don’t (or are not aware of it). So what really DOES happen when you clean out a snapshot? Time to investigate!

Test Setup

So how do we test performance impact on storage while ruling out external factors? The setup I choose was using a VM with the following specs:

Read the rest of this entry »

A hugely underestimated requirement in larger VDI environments is disk IOPs. A lot of the larger VDI implementations have failed using SATA spindles, when you use 15K SAS or FC disks you get away with it most of the times (as long as you do not scale up too much). I have been looking at ways to get more done using less (especially in current times, who doesn’t!). Dataman, the dutch company I work for (www.dataman.nl) teamed up with Sun Netherlands and their testing facility in Linlithgow, Scotland for testing. I got the honours of performing the tests, and I almost literally broke the sound barrier using Suns newest line of Unified Storage: The 7000 series. Why can you break the sound barrier with this type of storage? Watch the story unroll! For now part one… The intro.

What VMware View offers… And needs

Before a performance test even came to mind, I started to figure what VMware View offers, and what it needs. It is obvious: View gives you linked cloning technology. This means, that only a few full clones (called replicas) are read by a lot of Virtual Desktops (or vDesktops as I will call them from now on) in parallel. So what would really help pushing the limits of your storage? Exactly, a very large cache or solid-state disks. Read the rest of this entry »

No rocket science here. esxtop has always been there. Yet a lot of people miss out on some of its great features. Hopefully this blogpost will get you interested in looking at esxtop (again?) in detail!

Yesterday I attended a very interesting breakout session about esxtop and its advanced features in vSphere. Old news you might say, but there is SO much you can do with esxtop. For example, you can export data from esxtop and import them in Windows perfmon. And if you did know that, then for example, did you know you can now actually see which physic NIC is being used by a certain VM?

Other neat little features were shown. The best one being that the “swcur” field is actually NOT about the current swapping activity of a VM, but swapping that occured in the past (yes, I too would have called it differently…). How many of you knew that one? Finally, a very interesting field in the storage screen (yes for those who did not know that one, esxtop is not just about CPU, but also memory, storage, and new in vSphere… Interrupts) ). This field is called “DAVG” and this shows the actual latency seen by ESX to your storage (and also KAVG for kernel latency and GAVG for the total latency the guest sees).

There were also a few examples of misbehaving VMs which was very interesting to see. Numbers which seemed not possible, yet explained perfectly. I would like to vote this very last presentation at VMworld 2009 the best technical presentation I witnessed there!

I hope I got you (re)interested in esxtop. I am more of a graphical guy, so I like the performance monitor embedded within the VI client. But some things just aren’t there. So esxtop is definitely worth a(nother) look. If you’re using ESXi, make sure to download the vMA appliance (here) which has resxtop included (which looks a lot like esxtop on ESX).

Everybody does it – and if you don’t, you can’t play along. What am I talking about? Data deduplication. It’s the best thing since sliced bread I hear people say. Sure it saves you a lot of disk space. But is it really all that brilliant in all scenarios?

The theory behind Data Deduplication

The idea is truly brilliant – You store blocks of data in a storage solution, and you create a hash which identifies the data inside the block uniquely. Every time you need to backup a block, you check (using the hash) if you already have the block in storage. If you do, just write a pointer to the data. Only if you have not got the block yet, copy it and include it into the storage dedup Dbase. The advantage is clear: The more equal data you store, the more you save in disk space. This is, especially in VMware, using equal VMs from templates a very big saving in disk space.

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The actual dilemma

Certainly a nice thing about deduplication is, next to the large amounts of storage (and associated costs) you save, is that when you deduplicate at the source, you end up only sending new blocks across the line, which could dramatically reduce the bandwidth you need between remote offices and central backup locations. Deduplication at the source also means, you generally spread CPU loads better across your remote servers instead of locally in the storage solution.

Since there is a downside on every upside – Data Deduplication certainly has its downsides. For example, if I had 100 VMs, all from the same template, there surely are blocks that occur in each and every one of them. If that particular block gets corrupted… Indeed! You loose ALL your data. Continuing to scare you, if the hash algorithm you use is insufficient, two different data blocks might be identified as being equal, resulting in corrupted data. Make no mistake, the only way you can be 100% percent sure the blocks are equal, you need a hash number as big as the block itself (rendering the solution kind of useless). All dedup vendors use shorter hashes (I wonder why ), and live with the risk (which is VERY small in practice but never zero). Third mayor drawback, is the speed at which the storage device is able to deliver your data (un-deduplicated) back to you (which especially hurts on backup targets which have to perform massive restore operations). Final drawback: You need your ENTIRE database in order to perform any restore (at least you cannot be sure which blocks are going to be required to restore a particular set of data).

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So – should I use it?

The reasons stated above always kept me a skeptic when it came to data deduplication, especially for backup purposes. Because at the end of the day, you want your backups to be functional, and not requiring the ENTIRE dataset in order to perform a restore. Speed can also be a factor, especially when you rely on restores from the dedup solution in a case of disaster recovery.

Still, there are definitely uses for deduplication. Most vendors have solved most issues with success, for example being able to access un-deduplicated data directly from the storage solution (enabling separate backups to tape etc). I have been looking at the new version of esXpress with their PHDD dedup targets, and I must say it is a very elegant solution (on which I will create a blog shortly