From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: SSD data reliable vs. unreliable [Was: Re: Data Recovery from SSDs - Impact of trim?] Date: Tue, 27 Jan 2009 10:49:08 +0000 Message-ID: <497EE6A4.7030406@anonymous.org.uk> References: <87f94c370901221553p4d3a749fl4717deabba5419ec@mail.gmail.com> <497A2B3C.3060603@redhat.com> <1232749447.3250.146.camel@localhost.localdomain> <87f94c370901231526jb41ea66ta1d6a23d7631d63c@mail.gmail.com> <497A542C.1040900@redhat.com> <7fce22690901260659u30ffd634m3fb7f75102141ee9@mail.gmail.com> <497DE35C.6090308@redhat.com> <87f94c370901260934vef69a2cgada9ae3dfdb440ef@mail.gmail.com> <1232992065.3248.38.camel@localhost.localdomain> <18814.39074.194781.490676@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <18814.39074.194781.490676@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: James Bottomley , Greg Freemyer , Ric Wheeler , linux-raid , Dongjun Shin , IDE/ATA development list List-Id: linux-ide@vger.kernel.org On 27/01/2009 05:16, Neil Brown wrote: [...] > Probably the only practical data structure for this would be a bitmap > similar to the current write-intent bitmap. > > Is it really worth supporting this in raid5? Are the sorts of > devices that will benefit from 'discard' requests likely to be used > inside an md/raid5 array I wonder.... Assuming I've understood correctly, this usage map sounds to me like a useful thing to have for all RAIDs. When building the array in the first place, the initial sync is just writing a usage map saying it's all empty. Filesystem writes and discards update it appropriately. Then when we get failing sectors reported via e.g. SMART or a scrub operation we know whether they're on used or unused areas so whether it's worth attempting recovery. Cheers, John.