From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: additional feature for linear Date: Sat, 17 Sep 2011 17:03:41 +0200 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 17/09/11 09:12, Henti Smith wrote: > Good day, > > I have an itch I'm hoping somebody can provide me information with to > scratch it. Please be patient, I'm not very well versed in all the > technical information regarding RAID, so I'm still finding my way > around. > > Lets start with the itch. > > I'm looking for a way to take 3 drives and create one pool. This is > similar to linear mode, but in this case if a drive in the linear mode > fail the rest of the pool is intact and just the missing files are > removed from the "device" > > From my reading it looks like linear mode is the most likely > candidate, but there is no guarantee that the remaining data will be > accessible. > > "If one disk crashes you will most probably lose all your data. You > can however be lucky to recover some data, since the filesystem will > just be missing one large consecutive chunk of data" > > Would it not be possible to add functions to linear mode to mark this > missing chunks as "bad blacks" and let the FS deal with it as such > thereby allowing you to mount the linear device as per normal and > still access the remaining data ? > > Id this is not possible, is there not some other way to implement this ? > > I see there is some mails regarding adding additional drives to linear > as well and that the correct way is to stop the linear array and > recreate it with additional drives. Is this correct ? > > I also see there was work being done on extending the array while > online. Was this ever done ? > > Regards > Henti All this would require a filesystem that can cope with suddenly losing large numbers of disk blocks. Most file systems can't - so even if the raid layer simply marked the missing chunks as bad, the filesystem would die. What you are asking for here is a filesystem that understands the separate disks, and is careful to put individual files and related metadata and directories onto individual disks (so that when a disk dies, the file is either intact or completely lost), as well as duplicating its critical metadata so that it will survive a disk loss. All of this is the direct antithesis of raid, which aims to make multiple disks look like a single block device. I believe at the moment, your only answer (other than to re-think your requirements) is ZFS. It may be that btrfs has the features you need - they are certainly planned, as far as I know - but you'd have to be sure of using a recent kernel and utilities.