[SOLVED] FSTrim and Discard needed on ZFS?

jmcorrea

jmcorrea

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Jul 16, 2018
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Hello!

We have a couple of servers configured to use local ZFS pools for storage. These pools have Thin Provisioning enabled.
I used to think that Thin Provisioning + ZFS was enough to have the VMs use on physical storage the same space that they are using into themselves, but it seems I was wrong.
Just to check if I am right, Thin Provisioning only enables the virtual disks to grow according to the needs of the VM, but it does not shrink it, correct? Pretty much link Dynamic VHDs on Hyper-V.
If I want to have the VMs use the least physical disk space needed I have to set them with the Discard option enabled on they disks and also run fstrim periodically on them. Am I right?
And I have one more doubt, the disk space that is grown on the virtual disk but is not actually used on the VM, does it get replicated too? Or replication skips that space? If the "empty" space on the virtual disks is replicated then I guess shrinking the virtual disk should help to improve replication performance, correct?

Thank you very much, any advice is welcome!

Best regards,

Juan
 
Hi jmcorrea,

All thin allocated storages have to run fstrim inside the VM.
And if you run FStrim you must enable discard on the vdisk.
With this flag, FStrim is able to send the erased blocks to the storage layer.

lhorace said:
I just learned, ZFS discard/trim won't happen till 0.8?
Click to expand...
What you mean is trim for the ZFS itself.
This has nothing to to with thin allocated disks.
 
wolfgang said:
With this flag, FStrim is able to send the erased blocks to the storage layer.
Click to expand...

As long as this has not changed, you also need to use SCSI virtio in order to get TRIM to work.

jmcorrea said:
Just to check if I am right, Thin Provisioning only enables the virtual disks to grow according to the needs of the VM, but it does not shrink it, correct?
Click to expand...

No, that is exactly why you have to run TRIM inside of your guest (with all the settings @wolfgang and I said).
 
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