On 2013-01-24 11:06, Darren J Moffat wrote:

On 01/24/13 00:04, Matthew Ahrens wrote:
On Tue, Jan 22, 2013 at 5:29 AM, Darren J Moffat
<darr...@opensolaris.org <mailto:darr...@opensolaris.org>> wrote:

    Preallocated ZVOLs - for swap/dump.

Darren, good to hear about the cool stuff in S11.

Yes, thanks, Darren :)

Just to clarify, is this preallocated ZVOL different than the
preallocated dump which has been there for quite some time (and is in
Illumos)?  Can you use it for other zvols besides swap and dump?

It is the same but we are using it for swap now too.  It isn't available
for general use.

Some background:  the zfs dump device has always been preallocated
("thick provisioned"), so that we can reliably dump.  By definition,
something has gone horribly wrong when we are dumping, so this code path
needs to be as small as possible to have any hope of getting a dump.  So
we preallocate the space for dump, and store a simple linked list of
disk segments where it will be stored.  The dump device is not COW,
checksummed, deduped, compressed, etc. by ZFS.

Comparing these two statements, can I say (and be correct) that the
preallocated swap devices would lack COW (as I proposed too) and thus
likely snapshots, but would also lack the checksums? (we might live
without compression, though that was once touted as a bonus for swap
over zfs, and certainly can do without dedup)

Basically, they are seemingly little different from preallocated
disk slices - and for those an admin might have better control over
the dedicated disk locations (i.e. faster tracks in a small-seek
stroke range), except that ZFS datasets are easier to resize...
right or wrong?


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