On Fri, Oct 16, 2015 at 01:22:44PM -0600, Warner Losh wrote: > > > On Oct 16, 2015, at 7:19 AM, Slawa Olhovchenkov <s...@zxy.spb.ru> wrote: > > > > On Fri, Oct 16, 2015 at 03:06:02AM +0000, Warner Losh wrote: > > > >> Author: imp > >> Date: Fri Oct 16 03:06:02 2015 > >> New Revision: 289405 > >> URL: https://svnweb.freebsd.org/changeset/base/289405 > >> > >> Log: > >> Do not relocate extents to make them contiguous if the underlying drive > >> can do > >> deletions. Ability to do deletions is a strong indication that this > >> optimization will not help performance. It will only generate extra write > >> traffic. These devices are typically flash based and have a limited > >> number of > >> write cycles. In addition, making the file contiguous in LBA space doesn't > >> improve the access times from flash devices because they have no seek > >> time. > > > > In reality, flash devices have seek time, about 0.1ms. > > Many flash devices can do 8 simultaneously "seek" (I think NVMe can do > > more). > > That's just not true. tREAD for most flash is a few tens of microseconds. The > streaming time is at most 10 microseconds. There's no "seek" time in the > classic > sense. Once you get the data, you have it. There's no extra "read time" in > the NAND flash parts. > > And the number of simultaneous reads depends a lot on how the flash vendor > organized the flash. Many of today's designs use 8 or 16 die parts that have 2 > to 4 planes on them, giving a parallelism in the 16-64 range. And that's > before > we get into innovative strategies that use partial page reads to decrease > tREAD > time and novel data striping methods. > > Seek time, as a separate operation, simply doesn't exist. > > Furthermore, NAND-based devices are log-structured with garbage collection > for both retention and to deal with retired blocks in the underlying NAND. The > relationship between LBA ranges and where the data is at any given time on > the NAND is almost uncorrelated. > > So, rearranging data so that it is in LBA contiguous ranges doesn't help once > you're above the FFS block level.
Stream of random reads 512-4096 bytes from most flash SATA drives in one thread give about 10K IOPS. This is only 40Mbit/s from 6*0.8 Gbit/s SATA bandwidth. You may decompose 0.1ms to different, real delay (bank select, command process and etc.) or give 0.1ms seek time for all practical purpose. _______________________________________________ svn-src-head@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
