Hi, On Mon, 30 Jun 2008, Daniel Kahn Gillmor wrote:
> On Mon 2008-06-30 05:20:18 -0400, Gavin McCullagh wrote: > > > -- Consistency -- > > > > The trouble with RAID is what happens when you get a sudden system > > shutdown (eg due to a power failure) while you're writing to the > > disk array. Suppose the data is written to one drive correctly, but > > not yet to the other. When the system resumes, it's very difficult > > for the system to determine which disk (if either) is correct. So, > > you end up with some ambiguous blocks where depending on which disk > > is read, you can get a different answer. An FSCK may not fix or > > even detect the issue. > > > > Most software RAID (ZFS excepted) suffers from the above issue and > > some cheaper hardware RAID cards do too. > > Is this still true with modern mdadm? I thought that the RAID > superblock on modern systems contained information about > "last-written" times, to enable recovery from exactly this problem. > I'm not a RAID guru, though: if anyone could point me toward the > relevant documentation, i'd be grateful. I'm not aware that this has changed, but I could be wrong and would be delighted to find I am. Would "last-written" times be enough to recover from this? I would have thought you'd need a full-blown journal. > At any rate, RAID or not, for important machines you probably want to > have a UPS prevent sudden power failures, and you should also have a > real backup strategy. Indeed, though if you're on a budget, UPS is not always affordable and it won't prevent every sort of interruption (like a kernel panic). > RAID (protection against hardware failures) is not a substitute for > backups (protection against human error). Absolutely. RAID keeps uptime, not backups. Gavin -- edubuntu-users mailing list [email protected] Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/edubuntu-users
