Tim Cook wrote:
On Mon, Jan 18, 2010 at 3:49 PM, Richard Elling
<richard.ell...@gmail.com <mailto:richard.ell...@gmail.com>> wrote:
On Jan 18, 2010, at 7:55 AM, Jesus Cea wrote:
> zpool and zfs report different free space because zfs takes into
account
> an internal reservation of 32MB or 1/64 of the capacity of the pool,
> what is bigger.
This space is also used for the ZIL.
> So in a 2TB Harddisk, the reservation would be 32 gigabytes.
Seems a bit
> excessive to me...
Me too. Before firing off an RFE, what would be a reasonable upper
bound? A percentage?
-- richard
Not being intimate with the guts of ZFS, it would seem to me that a
percentage would be the best choice. I'll make the (perhaps
incorrect) assumption that as disks grow, if you have a set amount of
free space (say 5g), it becomes harder and harder to find/get to that
free space resulting in performance tanking. Where as we can expect
linear performance if it's a percentage. No?
--
--Tim
Actually, as a section of the reservation is ZIL, that portion's impact
on performance is directly tied to the PERFORMANCE of the underlying
zpool, not it's size. As such, given that hard drive performance is
pretty much hit a wall, I think we should look at having a
non-size-determined limit on the size of the reserved area, regardless
of the actual size of the zpool. The limit would still need to be
heuristic, since higher-performing zpools would need to have a larger
maximum reservation than lower-performing ones.
Given my (imperfect) understanding of the internals of ZFS, the non-ZIL
portions of the reserved space are there mostly to insure that there is
sufficient (reasonably) contiguous space for doing COW. Hopefully, once
BP rewrite materializes (I know, I'm treating this much to much as a
Holy Grail, here to save us from all the ZFS limitations, but
really...), we can implement defragmentation which will seriously reduce
the amount of reserved space required to keep up performance.
Once all this gets done, I'd think we seldom would need more than a GB
or two as reserve space...
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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