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. 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. 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: 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
