On Tue, Jan 22, 2013 at 5:29 AM, Darren J Moffat <darr...@opensolaris.org>wrote:
> Preallocated ZVOLs - for swap/dump.
>
Darren, good to hear about the cool stuff in S11.
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?
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.
In Illumos (and S10), swap was treated more or less like a regular zvol.
This leads to some tricky code paths because ZFS allocates memory from
many points in the code as it is writing out changes. I could see
advantages to the simplicity of a preallocated swap volume, using the same
code that already existed for preallocated dump. Of course, the loss of
checksumming and encryption is much more of a concern with swap (which is
critical for correct behavior) than with dump (which is nice to have for
debugging).
--matt
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