Quick post here. I am working on updating some documentation and I wanted to add a bit more color to a section on changing the IO Operations limit for ESXi NMP Round Robin devices. The Pure Storage recommendation is to change this value to one from the default of 1,000. Therefore, ESXi will switch logical paths after each I/O instead of 1,000. There are some performance benefits to this and some evidence for improved failover time (in the case of a path failure) with this setting. I am not going to get into the veracity of these benefits right now. What I wanted to share here is that there is no doubt changing this to 1 makes a big difference to I/O balance on the array itself. Continue reading ESXi IO Operations Limit Parameter and IO Balance
UPDATE: In-guest UNMAP is now supported in a VM and sDelete and such is no longer required. Please refer to these posts:
Direct Guest OS UNMAP in vSphere 6.0
Reclaiming “dirty” or “dead” space is a topic that goes by my desk quite often these days–since the FlashArray is a data reduction array it is especially important that space is not wasted on the array–throws off the economics etc. Therefore UNMAP is an important VAAI feature to leverage in any AFA environment. Supporting UNMAP is definitely table stakes for AFAs.
Note–I am doing to use the terms “dead”, “dirty” and “stranded” to define space that needs to be reclaimed interchangeably. So anyways…
Unfortunately UNMAP in its current form does not satisfy all of the reclamation use cases. UNMAP will only reclaim space on any array when capacity is cleared from the VMFS volume–so when a VM (or virtual disk) is deleted or migrated elsewhere. It does not have the ability to reclaim space when data is “deleted” inside a virtual machine by the guest OS when using virtual disks. VMware does not know this capacity has been cleared and neither can the array. So until this virtual disk is deleted or moved the capacity cannot be reclaimed with UNMAP. So to be clear, UNMAP with vmkfstools (in ESXi 5.0/5.1) or esxcli (in ESXi 5.5) does not allow you to reclaim space that remains stranded inside of virtual disks.
I posted a week or so ago about the ESXCLI UNMAP process with vSphere 5.5 on the Pure Storage FlashArray here and came up with the conclusion that larger block counts are highly beneficial to the UNMAP process. So the recommendation was simply use a larger block count than the default to speed up the UNMAP operation, something sufficiently higher than the default of 200 MB. I received a few questions about a more specific recommendation (and had some myself) so I decided to dive into this a little deeper to see if I could provide some guidance that was a little more concrete. In the end a large block count is perfectly fine–if you want to know more details–read on!