Date: Fri, 16 Oct 2015 23:18:50 +0300 From: Slawa Olhovchenkov <slw@zxy.spb.ru> To: Warner Losh <imp@bsdimp.com> Cc: Warner Losh <imp@FreeBSD.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r289405 - head/sys/ufs/ffs Message-ID: <20151016201850.GP6469@zxy.spb.ru> In-Reply-To: <3ADA7934-3EE1-449E-A8D1-723B73020C13@bsdimp.com> References: <201510160306.t9G3622O049128@repo.freebsd.org> <20151016131940.GE42243@zxy.spb.ru> <3ADA7934-3EE1-449E-A8D1-723B73020C13@bsdimp.com>
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On Fri, Oct 16, 2015 at 01:22:44PM -0600, Warner Losh wrote: > > > On Oct 16, 2015, at 7:19 AM, Slawa Olhovchenkov <slw@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.
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