how to set "Available Space Threshold" properly (ISCSI)

[Thomymaster]

Hi


Another question:

I have my vSphere 5.5 environment setup using a zVol based extent on my FreeNas box (still on 9.2.1.6 but i want to upgrade soon to 9.3, so here comes the question).


The 9.3 manual says (http://doc.freenas.org/9.3/freenas_intro.html#zfs-primer) that when using ISCSI, the pool (including i.e. datasets for CIFS) should never exceed 50%.

Then the ISCSI extent manual page says (http://doc.freenas.org/9.3/freenas_sharing.html#extents) that an extent should not exceed 50% for fragmentation.

OK, now i'm a bit confused.

Should i set both values to 50%?

Target Global Configuration -> Pool Available Space Threshold -> 50
Extents -> Properties of the ESXi extent -> Available Space Threshold -> 50


Cheers Thomy

 

[dlavigne]

Set it to a value that is useful to you. For example, you could set the threshold to 40% so that you receive the alert in plenty of time to arrange for additional capacity. Or you could set it to 50% to let you know that you should fix this now. Or to 60% to remind you that you really should have dealt with this by now. You get the idea

 

[Thomymaster]

But what counts now:

-should the pool (with all zVols and datasets) not exceed 50%
and/or
-any of the zVols used for ISCSI not exceed 50%

for performance reasons/fragmentation

 

[mav@]

From performance/fragmentation reasons you should mostly bother about "Pool Available Space Threshold". With one exception -- this global threshold does not work for file-backed extents, only for ZVOLs. So if you are using file-backed extents -- set per-extent thresholds for them.

When global threshold is working, extent's "Available Space Threshold" mostly protects against specific case when some other dataset/ZVOL has space reservation set so high, that even while pool still has plenty of available space, your ZVOL may be unable to write at some point. This value can be set lower, just to give you enough time to react before bad things happen.

And note, that those variables are called "Available Space Threshold", so their meaning is opposite to (at least my interpretation of) Dru's comment -- they specify percentage of FREE (available) space that should be left on pool/zvol, not a space percentage of used space.

 

[Thomymaster]

OK

As i'm only using zVol backed extents (to get the benefit of the VAAI support) and some datasets for CIFS, i think its the best to set the global threshold to 50% and not to use per-extend thresholds.

 

[mav@]

Per-extent thresholds would not hurt either. But you may set them to lower values. Per-extent thresholds don't override the global one. They work simultaneously.