VMFS Blocksizes have always been a hot topic regarding storage performance. It has been discussed by many including Eric Siebert on ITKE and Gabe also opened a topic on VMTN and he answered his own question at the bottom. Steve Chambers wrote a great article about Disk Alignment and Blocksize on VI:OPS which also clearly states:”the VMFS block size is irrelevant for guest I/O.” Reading these articles/topics we can conclude that an 8MB blocksize opposed to a 1MB blocksize doesn’t increase performance.
But, is this really the case? Isn’t there more to it than meets the eye?
Think about thin-provisioning for a second. If you create a thin provisioned disk on a datastore with a 1MB blocksize the thin provisioned disk will grow with increments of 1MB. Hopefully you can see where I’m going. A thin provisioned disk on a datastore with an 8MB blocksize will grow in 8MB increments. Each time the thin-provisioned disk grows a SCSI reservation takes place because of meta data changes. As you can imagine an 8MB blocksize will decrease the amount of meta data changes needed, which means less SCSI reservations. Less SCSI reservations equals better performance in my book.
For the current VI3 environments, besides VDI, I hardly have any customers using thin provisioned vmdk’s. But with the upcoming version of ESX/vCenter this is likely to change because the GUI will make it possible to create thin provisioned vmdk’s. Not only during the creation of vmdk’s will thin provisioned disks be an option, but also when you initiate a Storage VMotion you will have the option to migrate to a thin provisioned disk. It’s more than likely that thin provisioned disks will become a standard in most environments to reduce the storage costs. If it does, remember that when a thin provisioned disk grows a SCSI reservation takes place and less reservations is definitely beneficial for the stability and performance of your environment.
