An order of magnitude higher IOPS needed with ZFS than UFS

[Jeremy Chadwick]

On Wed, Jun 12, 2013 at 06:40:32AM -0500, Mark Felder wrote:
> On Tue, 11 Jun 2013 16:01:23 -0500, Attila Nagy <bra at fsn.hu> wrote:
> 
> >BTW, the file systems are 77-78% full according to df (so ZFS
> >holds more, because UFS is -m 8).
> 
> ZFS write performance can begin to drop pretty badly when you get
> around 80% full. I've not seen any benchmarks showing an improvement
> with a very fast and large ZIL or tons of memory, but I'd expect
> that would help significantly. Just note that you're right at the
> edge where performance gets impacted.

Mark, do you have any references for this?  I'd love to learn/read more
about this engineering/design aspect (I won't say flaw, I'll just say
aspect) to ZFS, as it's the first I've heard of it.

The reason I ask: (respectfully, not judgementally) I'm worried you
might be referring to something that has to do with SSDs and not ZFS,
specifically SSD wear-levelling performing better with lots of free
space (i.e. a small FTL map; TRIM helps with this immensely) -- where
the performance hit tends to begin around the 70-80% mark.  (I can talk
more about that if asked, but want to make sure the two things aren't
being mistaken for one another)

-- 
| Jeremy Chadwick                                   jdc at koitsu.org |
| UNIX Systems Administrator                http://jdc.koitsu.org/ |
| Making life hard for others since 1977.             PGP 4BD6C0CB |