From mboxrd@z Thu Jan 1 00:00:00 1970 From: Linda Walsh Subject: Re: RAID needs more to survive a power hit, different /boot layout for example (was Re: draft howto on making raids for surviving a disk crash) Date: Tue, 05 Feb 2008 04:31:29 -0800 Message-ID: <47A85721.2090807@tlinx.org> References: <47A612BE.5050707@pobox.com> <47A623EE.4050305@msgid.tls.msk.ru> <47A62A17.70101@pobox.com> <47A6DA81.3030008@msgid.tls.msk.ru> <47A6EFCF.9080906@pobox.com> <47A7188A.4070005@msgid.tls.msk.ru> <47A72061.3010800@sandeen.net> <47A72FBC.9090701@pobox.com> <47A7411F.2040702@sandeen.net> <47A749C9.6010503@msgid.tls.msk.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <47A749C9.6010503@msgid.tls.msk.ru> Sender: linux-raid-owner@vger.kernel.org To: xfs@oss.sgi.com Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Michael Tokarev wrote: > note that with some workloads, write caching in > the drive actually makes write speed worse, not better - namely, > in case of massive writes. ---- With write barriers enabled, I did a quick test of a large copy from one backup filesystem to another. I'm not what you refer to when you say large, but this disk has 387G used with 975 files, averaging about 406MB/file. I was copying from /hde (ATA100-750G) to /sdb (SATA-300-750G) (both, basically underlying model) Of course your 'mileage may vary', and these were averages over 12 runs each (w/ + w/out wcaching); (write cache on) write read dev ave TPS MB/s MB/s hde ave 64.67 30.94 0.0 sdb ave 249.51 0.24 30.93 (write cache off) write read dev ave TPS MB/s MB/s hde ave 45.63 21.81 0.0 xx: ave 177.76 0.24 21.96 write w/cache = (30.94-21.86)/21.86 => 45% faster w/o write cache = 100-(100*21.81/30.94) => 30% slower These disks have barrier support, so I'd guess the differences would have been greater if you didn't worry about losing w-cache contents. If barrier support doesn't work and one has to disable write-caching, that is a noticeable performance penalty. All writes with noatime, nodiratime, logbufs=8. FWIW...slightly OT, the rates under Win for their write-through (FAT32-perf) vs. write-back caching (NTFS-perf) were FAT about 60% faster over NTFS or NTFS ~ 40% slower than FAT32 (with ops for no-last-access and no 3.1 filename creation)