Disk Alignment

[DMcCoy]

I'm just wondering if anyone else out there aligns their disk partitions? This is most applicable to virtual machines and Exchange stores but applies to SANs and normal RAID arrays, along with various other OS.

 

It's to do with starting partitions on the 63rd sector, and how it doesn't align with the disk stripes. This means reads are often unnecessarily done across stripe boundaries.

 

In Vista, it is done for you, I assume this will apply to 2008 server too. XP and 2003 do not, I don't think linux does either.

 

There is a minimal, but real, performance hit of a few percent when the partition is not aligned. I came across this with my migration to ESX 3 as I hadn't aligned my partitions when my SAN was first set up (not to be confused with the SANs own alignment options!)

 

If you create your VMFS partions on the LUN (or RAID) volume with Virtual Center 2.0 (or 2.5) then it will align to a 32k boundary for you, the console and scripted install do not (nor will the installer).

 

Not only do the VMFS partitions have to be aligned, the virtual machine virtual disks need to be aligned too! This involves creating the disk on an existing server 2003 machine and using diskpar (not diskpart) to create an alligned partition, this disk is then assigned to the correct VM for use.

 

It was quite a lot of work as its a destructive process and I had to move the data between my LUNs to eventually repartition them all!

 

I'm not telling people "go and do this!" I'm simply curious with the larger number of ESX users we have now. (http://www.vmware.com/pdf/esx3_partition_align.pdf for the curious).

 

David

Edited February 1, 2008 by DMcCoy
Fine, I'll make a more interesting title!

[Geoff]

I use GNU parted for partition creation. It automatically aligns the file system partitions to the correct boundaries..

[PiqueABoo]

I've done that on Exchange boxes, but have also encountered a somewhat plausible claim that alignment may do very little for disks nowadays because of all the magic they do behind the scenes to get those lovely big modern capacities i.e. the text-book disk geometry model is virtual, not what really happens on the disk.

 

I'll let someone else figure out which story is correct.

[DMcCoy]

I use GNU parted for partition creation. It automatically aligns the file system partitions to the correct boundaries..

 

Ubuntu server doesn't align correctly when installing

 

I've done that on Exchange boxes, but have also encountered a somewhat plausible claim that alignment may do very little for disks nowadays because of all the magic they do behind the scenes to get those lovely big modern capacities i.e. the text-book disk geometry model is virtual, not what really happens on the disk.

 

I'll let someone else figure out which story is correct.

 

It's about making additional stripe crossings with raid (http://support.microsoft.com/kb/929491 has some info)

 

My testing shows a increase of peak throughput of around 6% with my SAN.