ZFS iSCSI and free capacity in VMFS

[vinhdat82]

Hi everyone,

I have the same issue as in http://communities.vmware.com/thread/188141

I use ZFS to provide iSCSI LUN to Vmware ESXi 5.0.

However, the old SAN doesn't have iSCSI unmap. I have create / move / delete virtual machine between ZFS iSCSI. After a while, zpool list report 95% capacity used. However, Vmware shows VMFS only uses 50% capacity.

AFAIK, when creating a virtual machine, zvol allocated disk space.
However, when we moved / deleted virtual machine, zvol didn't update / didn't aware to update the free capacity.

Now, I believe due to the wrong free capacity reported in zpool list, the performance is deeply affected.

How I should do to let ZFS know the real free capacity.

I followed the trick in the post in iSCSI LUN from ESXi

dd if=/dev/zero of=testfile bs=1024k count=50000; sync;sleep 1;sync;rm -f testfile

However the trick is not working.

How I could let zfs know the true free capacity in VMFS? Zero-out VMFS?

Thanks million.

[mcowger]

zfs will never know the true usage, because the filesystem (VMFS) doesn't have any way to inform the volume (ZFS) that a block that supported a deleted file is no longer in use.

There is a feature to do this (the UNMAP command), but ZFS does not yet support this SCSI command.

Without UNMAP, you are out of luck.

[vinhdat82]

Is there any workaround?

As the performance decreases when zfs realize free% decreases (wrongly).

I only want to zero-out or let zfs know the true free% so the performance increases and I could move VMs somewhere else faster.

[mcowger]

Create new volume.  svmotion all VMs to new volume.  Delete old volume.

[vinhdat82]

We have allocated all capacity in the server.

And it is too slow to storage vmotion due to the wrong free capacity in ZFS.