VMFS Partition Alignment

[JustyC]

We have a small and somewhat new virtual environment. Most guests are Windows with a few Linux. Now hear there is a partition alignment issue with VMFS regardless of storage vendor. Can anyone offer any insight as to whether this should be a concern (performance in mind) - should the guests we already ctreated be realigned? We have other physical servers connected to a EMC SAN and although we changed the starting block to 128 we did not see any differenec in performance. If realignmemnt is recommended then what is the best way to do that?

[Troy_Clavell]

have a look at this

You can use vOptimizer to change your disk alignments of your guests.

If you present your LUNS using the VIC, then the LUNs will be aligned correctly.

[kastlr]

Hi,

the problem with misaligned partitions is mostly windows related.

If you didn't create the VMFS datastores from the service console CLI, it will always be aligned.

Problem is within the vmdk files created for your windows OS (except W2K8), if you don't use diskpart the partitions aren't aligned.

Check the following paper available from EMC² (you do need a powerlink account).

Using diskpar and diskpart to Align Partitions on Windows Basic and Dynamic Disks

You could expect a maximum performance win from 10-12%, but that depends on the IO workload.

Keep in mind that SAN arrays are focused on multiple threads, a single copy job won't be a valid performance test.

Hope this helps a bit.

Greetings from Germany. (CET)

[JustyC]

Will have to review all the VM's we created so far (40 - 50). Most are W2K3 - a few have SQL2005 - these and a few others may have high IO's, which might make them good candidates for realignment. The VMWare article "Recommendations for Aligning VMFS Partitions" says that aligning virtual machine boot disks is not recommended or required. Regardless, most of our VMs have 2 or more partitions for applications or data. These (Windows) partitions are usually configured as dynamic not basic disk. Ultimately want to move all guests from local disk or our HP SAN to a NetApp FAS2050 and realign what really needs to be realigned. There is a utility from NetApp (mbralign) that will correct the alignment, but the VMs may need to be powered off and I don't think it works with dynamic disks. Correcting the alignment could be painful.

[Matt_B1]

We also found contradicting advice on aligning our partitions. In the end, regardless of VMDK or RDM, we leave the boot partition with no offset and as a basic disk. We have all Win 2k3 VMs. For all other partitions, we use diskpar.exe and create a 16 MB partition with a starting offset of 128 sectors. We then format the rest of the disk for data and set the whole disk as dynamic. We have not done any performance tests on this but we found this to be the most agreed upon for our Win 2k3 VMs with EMC Symmetrix storage setup.

[AndreTheGiant · Accepted solution]

See also:

Andre