From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Campbell Subject: Re: Why does the md/raid subsystem does not remap bad sectors in a raid array? Date: Sun, 23 Nov 2008 08:34:20 +0400 Message-ID: <4928DD4C.4020301@wasp.net.au> References: <4928B580.5040800@shaw.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4928B580.5040800@shaw.ca> Sender: linux-raid-owner@vger.kernel.org To: Robert Hancock Cc: linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-raid.ids Robert Hancock wrote: >> The controller does not drop the drive from the array when it hits an >> error, the 3ware card "takes care of it" and the user need not worry >> about it, whereas with md/raid every time it hits a bad sector, it >> breaks the raid and it goes degraded, is this correct? Will/can >> something like what 3ware does be possible in a sw-raid based >> configuration or is a HW raid card required? > > Presumably all it's doing is writing that sector's contents back from > the other drive(s) in the array when the read error is detected, this is > something that software could do just as well. Drives only remap bad > sectors when they are written over, as a read failure doesn't > necessarily mean that the sector is entirely unreadable, but could be > due to environmental factors such as high temperature, vibration, etc. > > Just rewriting the sector seems a bit questionable though, as if a drive > in your array is growing read errors that's not really a good thing.. md has done this for a while now though. If it encounters a read error in the array it will make an attempt to write the reconstructed data back to that disk attempting to force a reallocation. I've seen it work quite well here on disks that have the occasional grown defect. It's certainly _much_ nicer than having the disk booted from the array on a single read error. If the disk is haemorrhaging sectors then you will find out about it sooner or later through other means. Brad -- Dolphins are so intelligent that within a few weeks they can train Americans to stand at the edge of the pool and throw them fish.