Last week I wrote about the different datamovers being used when a Storage vMotion is initiated and the destination VMFS volume has a different blocksize as the source VMFS volume. Not only will it make a difference in terms of reclaiming zero space, but as mentioned it also makes a different in performance. The question that always arises is how much difference does it make? Well this week there was a question on the VMTN community regarding a SvMotion from FC to FATA and the slow performance. Of course within a second FATA was blamed, but that wasn’t actually the cause of this problem. The FATA disks were formatted with a different blocksize and that cause the legacy datamover to be used. I asked Paul, who started the thread, if he could check what the difference would be when equal blocksizes were used. Today Paul did his tests and he blogged about it here and but I copied the table which contains the details that shows you what performance improvement the fs3dm (please note, that VAAI is not used… this is purely a different datamover) brought:
From To Duration in minutes FC datastore 1MB blocksize FATA datastore 4MB blocksize 08:01 FATA datastore 4MB blocksize FC datastore 1MB blocksize 12:49 FC datastore 4MB blocksize FATA datastore 4MB blocksize 02:36 FATA datastore 4MB blocksize FC datastore 4MB blocksize 02:24
As I explained in my article about the datamover, the difference is caused by the fact that the data doesn’t travel all the way up the stack… and yes the difference is huge!