From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michael Evans Subject: Re: unbelievably bad performance: 2.6.27.37 and raid6 Date: Mon, 2 Nov 2009 22:39:49 -0800 Message-ID: <4877c76c0911022239q5af451d7o6739d597e6de06a0@mail.gmail.com> References: <4AEEF29C.10309@tmr.com> <9e52db7bed791d8c8f5653b1958b1874.squirrel@neil.brown.name> <4877c76c0911022209g60e40364i2cb82909e31f707d@mail.gmail.com> <763d89fcf9c0164cbe8b6245dc3d2d7f.squirrel@neil.brown.name> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <763d89fcf9c0164cbe8b6245dc3d2d7f.squirrel@neil.brown.name> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: Jon Nelson , LinuxRaid List-Id: linux-raid.ids > > A reshape is a fundamentally slow operation. =A0Each block needs to > be read and then written somewhere else so there is little opportunit= y > for streaming. > An in-place reshape (i.e the array doesn't get bigger or smaller) is > even slower as we have to take a backup copy of each range of blocks > before writing them back out. =A0This limits streaming even more. > > It is possible to get it fast than it is by increasing the > array's stripe_cache_size and also increasing the 'backup' size > that mdadm uses. =A0mdadm-3.1.1 will try to do better in this respect= =2E > However it will still be significantly slower than e.g. a resync. > > So reshape will always be slow. =A0It is a completely different issue > to filesystem activity on a RAID array being slow. =A0Recent reports = of > slowness are, I think, not directly related to md/raid. =A0It is eith= er > the filesystem or the VM or a combination of the two that causes > these slowdowns. > > > NeilBrown > stripe_cache_active =3D default 0? stripe_cache_size =3D default 256? (1mb per disk) cat /sys/block/md52/md/stripe_cache_* 0 8192 I bumped it up, but I haven't seen a real increase in the speed yet. Do I need to mdadm -S /dev/md52 and then re-assemble it to have this take effect, or is it the fact that the stripe cache active seems to be 'deactivated'? Can I safely set this to 1 mid-reshape? -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html