On Sun, 5 Feb 2012, Daniel Pittman <daniel@rimspace.net> wrote:
Yes, it does good. For something like RAID-5 that has enough information to detect which device is wrong you can find and correct problems with the system. With a RAID-1 you can know that a device is returning bad data.
You mean RAID-6. RAID-5 is no better than RAID-1 when it comes to determining which one of the disks is returning corrupt data. On Sun, 5 Feb 2012, Daniel Pittman <daniel@rimspace.net> wrote:
Yes. This is the substantial advantage that BTRFS and ZFS have over device-level RAID and LVM. The stronger checksums are useful because they can be checked inline with lower I/O cost, and the intimate knowledge of allocated space means you can check more quickly. Both very attractive features.
https://btrfs.wiki.kernel.org/ The above URL lists mirroring as "Additional features in development, I don't think I can use this on serious servers for a while. https://btrfs.wiki.kernel.org/articles/u/s/i/Using_Btrfs_with_Multiple_Devic... mkfs.btrfs in Debian/Unstable apparently supports RAID (the above URL has background information), so I will test it out. On Sun, 5 Feb 2012, James Harper <james.harper@bendigoit.com.au> wrote:
One thing it does do for you is 'touch' unused blocks, and finding that those are bad now rather than later is better IMO. Also, verifying consistency and finding that you have a silent corruption problem early can only be a good thing. This is especially important for RAID5 without battery backed write cache as it can detect the RAID5 write-hole (http://en.wikipedia.org/wiki/RAID_5_write_hole). Maybe write-intent bitmaps get around this these days though?
The only difference between the RAID-5 write hole and the issue of mismatched RAID-1 devices is what happens on a device failer. It's a lot worse for RAID-5 so that's a good reason for doing such checks on a RAID-5 array, or just using RAID-6. RAID-6 allows detecting and correcting the situation where one disk has corrupt data and all disks are working and it allows detecting the situation where the disks aren't consistent if there has been a single disk failure. Just don't use RAID-5. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/