On Friday 25 May 2007 03:35:48 Pallai Roland wrote: > On Fri, 2007-05-25 at 10:05 +1000, David Chinner wrote: > > On Thu, May 24, 2007 at 07:20:35AM -0400, Justin Piszcz wrote: > > > On Thu, 24 May 2007, Pallai Roland wrote: > > > >It's a good question too, but I think the md layer could > > > >save dumb filesystems like XFS if denies writes after 2 disks are > > > > failed, and > > > >I cannot see a good reason why it's not behave this way. > > > > How is *any* filesystem supposed to know that the underlying block > > device has gone bad if it is not returning errors? > > It is returning errors, I think so. If I try to write raid5 with 2 > failed disks with dd, I've got errors on the missing chunks. > The difference between ext3 and XFS is that ext3 will remount to > read-only on the first write error but the XFS won't, XFS only fails > only the current operation, IMHO. The method of ext3 isn't perfect, but > in practice, it's working well. Sorry, I was wrong: md really isn't returning error! It's madness, IMHO. The reason why ext3 safer on raid5 in practice is that ext3 remounts to read-only on read errors too and when a raid5 array got 2 failed drives and there's some read, the error= behavior of ext3 will be activated and stops further writes. You're right, it's not a good solution and there should be read operations to prevent data loss in this case on ext3 too. Raid5 *must deny all writes* when 2 disks failed: I still can't see a good reason why not, and the current method is braindead! > > I did mention this exact scenario in the filesystems workshop back > > in february - we'd *really* like to know if a RAID block device has gone > > into degraded mode (i.e. lost a disk) so we can throttle new writes > > until the rebuil dhas been completed. Stopping writes completely on a > > fatal error (like 2 lost disks in RAID5, and 3 lost disks in RAID6) > > would also be possible if only we could get the information out > > of the block layer. Yes, it's sounds good, but I think we need a quick fix now, it's a real problem and easily can lead to mass data loss. -- d - 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
