Hi Chip, On May 27, 2014, at 7:31 AM, Schweiss, Chip via illumos-discuss <[email protected]> wrote:
> > On Tue, May 27, 2014 at 8:46 AM, Bob Friesenhahn > <[email protected]> wrote: > On Tue, 27 May 2014, Schweiss, Chip wrote: > > Actually, with ZFS more free pages, does improve throughput, depending on how > aggressive the SSDs garbage collector is. > Since ZFS will write to every available block on the disk, after filling > once, every write afterwards is overwriting a > previously written logical block. If the SSD has enough spare blocks and an > aggressive enough garbage collector, there > > The above statement is contrary to what Richard Elling said about zfs > behavior. Richard Elling said that zfs allocates from lower-addressed > sectors first and so a non-full pool is unlikely to ever use upper-addressed > sectors. > > It is of course easy to test actual zfs behavior using a dtrace script. > > This is getting a bit off topic but quite important as to the reason you > would want to under provision an SSD. > > I have no doubt, Richard, knows the internals of ZFS better than me. I have > however done several long running IOzone tests against Samsung 840 Pro SSDs > sliced at 10% intervals between 60% and 100%. IOzone only allocated a > small percentage of the total. It was always clear at 90% and 100% when the > total pool capacity had been written as performance would drop off, no matter > what the write size. Between each run, all the SSDs were secure erased, > re-partitioned and allowed to sit idle for 1 hour so that all blocks were > zero when starting. Interesting results. Were you testing the "disk" or "disk with ZFS"? > > Below 80% and only small write size tests would outrun the garbage collector. > Even at 60% 4k random writes overran the the garbage collector and > performance tanked. I never let a 4k test run to completion because they > ran for over 6 hours when larger block size tests would complete in less than > 1 hour. > > Any SSD pool, knowing the work load and characteristics of the SSD being used > is paramount to tuning it to the best performance. Never expect the > manufactures published performance of any SSD for real workloads. SNIA's SSS-PTS is quite good at characterizing random block device performance. Some vendors publish SSS-PTS results, encourage more to do so :-) -- richard > > Back to the original question, slicing while not intuitive when first > encountered, does the job of keeping a reserved space from ZFS. I used the > following format script to create an EFI partition and slice the SSD. Adjust > to your particular SSD: > > fdisk > n > 1 > 1 > 80 > y > 6 > q > > -Chip > > > > illumos-discuss | Archives | Modify Your Subscription -- [email protected] +1-760-896-4422 ------------------------------------------- illumos-discuss Archives: https://www.listbox.com/member/archive/182180/=now RSS Feed: https://www.listbox.com/member/archive/rss/182180/21175430-2e6923be Modify Your Subscription: https://www.listbox.com/member/?member_id=21175430&id_secret=21175430-6a77cda4 Powered by Listbox: http://www.listbox.com
