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5 Replies Last post: May 13, 2008 2:23 AM by Gabrie  

VMFS and blocksize SAN performance? posted: May 6, 2008 3:26 AM

Click to view Gabrie's profile Master vExpert
Gabrie
902 posts since
Jun 6, 2005
Hi

For a particular VM I need 2x 1,6Tb disk. So I thought of creating two LUNs of 2Tb each with a 4Mb blocksize and put the VMDKs on them. Now I'm having a discussion with the SAN admin about the blocksize. We both agree that ESX can do sub-blocks, so if you have a 32kb file on you VMFS volume, it will not occupie a whole block of 4Mb, which is very nice.

But when a VM, acting as a file server, with VMDK on a 4Mb block VMFS volume, has to read files, what size is the block that will be read from SAN? Will this VM cause 4Mb block reads from the SAN even for very small files? This would cause quite a stress on our SAN, wouldn't it?

Gabrie

http://www.GabesVirtualWorld.com

Tags: san, vmfs, blocksize, storage_performance

Re: VMFS and blocksize -> SAN performance?

1. May 6, 2008 1:44 AM in response to: Gabrie
Click to view Dave.Mishchenko's profile Guru User Moderators vExpert
Dave.Mishchen…
9,151 posts since
Nov 15, 2005
Depending on your allignment, it will read 1 or more VMFS blocks. See page 5/6 here - http://www.vmware.com/pdf/esx3_partition_align.pdf.

Re: VMFS and blocksize SAN performance?

2. May 6, 2008 3:27 AM in response to: Dave.Mishchen…
Click to view Gabrie's profile Master vExpert
Gabrie
902 posts since
Jun 6, 2005
Hmm, so reading a 5kb file from the VMDK, would mean a 4Mb block from the SAN (if properly alligned). Doesn't this cause a big performance penalty???


http://www.GabesVirtualWorld.com

Re: VMFS and blocksize SAN performance?

3. May 6, 2008 6:24 AM in response to: Gabrie
Click to view kjb007's profile Guru vExpert
kjb007
5,618 posts since
Sep 18, 2006

Your scenario will be slightly worse. To create a 2 TB volume, you have to use an 8 MB block size, 4 MB will only allow 1 TB. And yes, you will have to read bigger chunks of data to get what you want, so there is a bit of a performance hit, but the penalty is usually offset by the cache to have on your array. You will hit more of a penalty if you have multiple servers that need access to the same block, and so will wait until the first read is done before having access to it.

-KjB

Re: VMFS and blocksize SAN performance?

4. May 13, 2008 1:40 AM in response to: Gabrie
Click to view BUGCHK's profile Master
BUGCHK
953 posts since
Nov 7, 2005

what size is the block that will be read from SAN?


I've just done a copy of a folder tree between two VMDKs and checked the I/O sizes with the EVA's performance utility (EVAperf).

Sometimes I saw a few I/Os with transfer sizes larger than 512KB - most sizes are between 4KB and 128KB. I even saw sizes smaller than 4KB.


Honestly, that did not surprise me ;-)

Re: VMFS and blocksize SAN performance?

5. May 13, 2008 2:23 AM in response to: BUGCHK
Click to view Gabrie's profile Master vExpert
Gabrie
902 posts since
Jun 6, 2005

Hi

Great answer bugcheck!! Last friday I called VMware Support to get some info on this. And they explained that the blocksize of the guest filesystem is the blocksize that will be read. The VMFS blocksize is not the blocksize that will be read from the SAN !!!!

The blocksize does has some impact when seeking begin and end of a block / chunk. That is what the partition alliging document refers to. Its about positioning the heads, NOT reading the blocks. So, if you have a NTFS file system in your guest, with only 32kb blocks, then 32kb blocks will be read from the SAN.

Gabrie

http://www.GabesVirtualWorld.com

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