UNMAP Primitive ESXI 5.5 and 6.0

[adamjs83]

I come across topics and articles stating that esxi 5.5 and 6.0 as well as freenas 9.3 support the UNMAP Primitive for iscsi however this topic is not covered anywhere in detail, at least in my searching. From what I gather when you run UNMAP from esxi it tells freenas to release the bits which were previously taken up by now deleted files. Supposedly this frees up space and reduces fragmentation. Can one conclude from this that if UNMAP is used regularly that it negates the need to keep the extent usage below 50% since the fragmentation is already being reduced?

 

[mav@]

Fragmentation is actually reduced by freeing more space on a pool. So you should still have about 50% of pool free, but with UNMAP used that does not mean that size of your iSCSI share should be only 50% of pool size.

You may even create ZVOL bigger then your pool size -- it just requires additional monitoring to avoid real pool overflows. To help with that FreeNAS supports two other VAAI primitives: Space threshold warnings and Stun.

 

[adamjs83]

that does not mean that size of your iSCSI share should be only 50% of pool size.

So what does it mean exactly? In a homelab scenario with ESXI 6.0 how does one manage how much VM data I can put on a datastore which is the only user of the pool without running in to performance issues?

 

[mav@]

There is no strict rules here. 50% is only a general recommendation. But even that is not very specific, since it goes about 50% of real pool usage, not about space reservation created for iSCSI disk. For example, if your VM stores data that are well compressed, you may create iSCSI disk bigger then 50% of pool size, or even bigger then whole pool size, and still be fine on performance.

In any case UNMAP gives you additional flexibility there -- when you delete some VM, its space will be freed on ZFS pool. Without UNMAP, if you created iSCSI disk bigger then 50% of pool size -- you are doomed: sooner or later during active use all blocks of iSCSI disk will be written at least once and occupy some space, even if they are not used by any VM any more. But with UNMAP that is not a problem -- after you deleted VM you can return its space back to the pool, no longer depending on iSCSI disk size at all.

 

[adamjs83]

Thanks, that is more clear now.

Just to clarify one thing, when you say "iSCSI disk" are you referring to the extent created in Freenas or the datastore created in esxi?

 

[mav@]

Just to clarify on thing, when you say "iSCSI disk" are you referring to the extent created in Freenas or the datastore created in esxi?

Both. Theoretically it is indeed possible to create datastore smaller then size of extent, but what for? I never did that.

 

[adamjs83]

Thanks so much for your responses.