On Wed, Jan 18, 2017 at 11:02 AM, Adam Richmond-Gordon <
[email protected]> wrote:

> I am assuming you skip the defrag section of this, or will sdelete only
> zero out the last contiguous block of “free” space?
>

Should be safe to skip the defrag. Layout on the virtual disk is probably
only loosely related to the layout of the zvol in the pool which is much
more about when the blocks were written, not where.


> I only ask as we snapshot our KVM zones frequently, so any kind of defrag
> would probably fill the zpool pretty quickly!
>

Be aware that while you can clean up the current state of a zvol, if you
don't delete old snapshots, you're not actually getting your storage back.
Take a look at "usedsnap" on the zvol to get a sense of how much your
snapshots are costing you, and look at the "used" on each snapshot to see
how much each is uniquely saving.
I did a very short writeup about how to figure out how much is currently
being held by more than one snapshot here:
https://blog.shalman.org/used-space-hidden-in-snapshots/

-Nahum



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