On Mon, May 26, 2014 at 12:01 PM, Bob Friesenhahn via illumos-discuss < discuss@lists.illumos.org> wrote:
> On Mon, 26 May 2014, Jim Klimov via illumos-discuss wrote: > >> >> Having an abundance of empty pages in advance in is one way to improve >> ssd write speeds, as well as longevity, and higher vendor over-provisioning >> is one way to achieve this. Other ways include the user overprovisioning >> that keeps some ranges of LBAs from ever being written. >> > > An abundance of empty pages improves write latency at lower write rates > but does not necessarily improve overall throughput since throughput is > still gated by page erase rates. > > The correct answer for what to do depends on how the device will be used > and on the device itself. > 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 will always been free physical blocks write to while garbage collecting the 'overwritten' logical block. Depending on the write size, write amplification will kill throughput, because there will not be enough complete pages available and the SSD will be forced into a lot more COW operations. However, if you are willing to allocate a lot of spare space, even small write sizes can be made to perform very well. The Anandtech article is a good reference to which SSDs do better at garbage collection than others. -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
