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.
-- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
