From mboxrd@z Thu Jan 1 00:00:00 1970 From: Asdo Subject: Re: Swapping a disk without degrading an array Date: Mon, 25 Jan 2010 15:51:27 +0100 Message-ID: <4B5DAFEF.7090500@shiftmail.org> References: <1264421475.30742.49.camel@test.apertos.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-reply-to: <1264421475.30742.49.camel@test.apertos.eu> Sender: linux-raid-owner@vger.kernel.org To: =?UTF-8?B?TWljaGHFgiBTYXdpY3o=?= Cc: linux-raid List-Id: linux-raid.ids Micha=C5=82 Sawicz wrote: > ... I wanted to start a discussion whether this at all makes sense, w= hat can > be the use cases etc. ... This appears just a great feature to me, you get my vote. I also was thinking about something similar. This is probably the most=20 desirable feature request for MD for me right now. Use cases could be: - 1 - the obvious one: you are seeing some preliminary errors=20 (correctable read errors, or SMART errors) on the disk and you want to=20 replace it without making the array degraded & temporarily vulnerable. - 2 - recoverying from a really bad array having multiple read errors i= n=20 different places in multiple disks (replacing one disk at a time with=20 the feature you suggest): consider that while filling each sector of th= e=20 the hot-spare the algorithm has 2 places where to read data from:=20 firstly it can try read from the drive being replaced, and then if that= =20 one returns read errors it can get the information from parity.=20 Currently there is no other way to do this with this level of redundanc= y=20 AFAIK, at least not automatically and not with the array online.=20 Consider that if you have a bad array as described, doing a full scrub=20 would take the array down, i.e. the scrub would never successfully=20 finish, and the new drive could never be filled with data. While with=20 the feature you suggest, there is no scrub on the whole array: data is=20 taken only from the drive being replaced for all the sectors (that's th= e=20 only disk being scrubbed), except possibly for a few sectors being=20 defective on that disk, for which parity is used. Thank you Asdo -- 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