VMware Solutions Discussions

Question regarding VAAI and alignment


Hello all,

I've got VAAI deployed for NFS datastores and I understand that without VAAI linked clones would be misaligned.  But what I'm not sure of is if the template from which the clones are created also needs to be aligned?  Does it even matter?  If my RHEL5u6 template is misaligned but I'm creating VAAI-based linked clones are there still misalignment issues I'll be dealing with?

And if so, can I use the VSC alignment tool to first fix the template's alignment issue?  Then subsequent VAAI clones would be good?  Thanks!




Hi Dave--

   Great questions!  So native linked clones on VMware ESX are delta files, just like their snapshots.  Technically speaking they CAN generate misaligned I/O (and almost always do), but let me give a brief explanation of why.  The grain size of linked clones (up until newest versions of ESX) was 512 bytes.  Because of this, you could have a write to the delta file that was as small as 512 bytes.  Since 4K is the smallest size physical block ONTAP writes in, if you have a 512 byte write, you have what is known as a partial write.  Partial writes, like misaligned I/O, incurs extra work within the system so it can lead to performance degradation. In ADDITION, because you shift the I/O over by that amount, subsequent writes are going to be misaligned as well, at least until you get lucky and have a write that puts you back on a 4K block boundary, but it is only a matter of time before another non-4K write comes in and starts the process over.

So back to your questions - yes, the template should definitely be aligned.  The I/O to the linked clone delta files created from that template will be no different than I described above, but we still HIGHLY recommend you align your templates, which will become apparent when I describe some alternatives in a moment.  The VSC alignment tool will have no affect on linked clones again because of the architecture as described above.

Now, for some alternatives.  NetApp will begin to recommend that you upgrade to VMware View 5.2 because it now has the ability to do one of two things. First, the SE Sparse disk format.  This new feature which requires View 5.2 and ESXi 5.1 eliminates the issues as described above because it has a 4K grain size and will result in constantly aligned I/O to the clone files.  The second option is if you are on NFS Datastores, to use the offloaded linked clones (VCAI), which basically allows View Composer to still create linked clones, but instead of VMware delta files, they result in NetApp FlexClones.  We are currently certifying under the VCAI program and will have that certification soon.  Both of these options are excellent alternatives to traditional linked clones.  NetApp will be publishing a lot of material around this soon, so stay tuned!