On Fri, Nov 23, 2007 at 02:45:37PM +0100, Christian Walther wrote: > Hi, > > On 23/11/2007, Jan Catrysse <[EMAIL PROTECTED]> wrote: > > > From: Bill Moran [mailto:[EMAIL PROTECTED] > > > Sent: Thursday, November 22, 2007 9:57 PM > > > To: Jan Catrysse > > > Cc: email@example.com > > > Subject: Re: RAID1 synchronisation - howto OR not necessary? > > > > > > "Jan Catrysse" <[EMAIL PROTECTED]> wrote: > > > > > [...] > > This is what I found in a 3Ware manual: > > Verification can provide early warning of a disk drive problem or failure. > [...] > Yes, it deals with drives failing silently. It's possible that data on > a disk can get corrupted. You don't see the error until you read the > block the next time. AFAIK the data contained in the bad block is > delivered to the CPU/RAM and can cause some damage there. > The problem I see with this setup is that you can't know which block > is the one that is okay. You need some additional feature like a > checksum for each block to check which one is correct. This isn't > supported by all file systems, so you can only see that there is a > difference between two blocks which sould normally be identical.
The disks themselves handle the checksumming to detect bad blocks. With modern disks it is *very* rare that a block on the disk goes bad without the disk being able to report it it as such. This means that if you have a functioning RAID1 setup and one of the disks report a bad block, then the controller can simply read the corresponding block from the other disk, and rewrite it to the disk with the bad block. If a disk has problems writing a block it will transparently re-map the block to another. The problems can occur when one disk in a RAID-array has failed and you try to rebuild it from the other disk(s). If you then encounter a bad block on that disk you have a problem since you don't have a good copy of that block. This is what verification (which, btw, is not the same as synchronization) tries to prevent by reading every block on each disk on a regular basis. Then the RAID controller can recover the data on any bad blocks from the other disk(s) in the array. > > I doubt that cheep RAID-Controllers have this capability, so you have > to stick to some software solution. Does FreeBSD take care about > something like this, I mean for software RAIDs? The cheap RAID-controller do almost all the RAID-operations in software anyway so it does indeed have to be a software solution anyway. I don't think FreeBSD's software RAID implementations have any support for disk verification - at least I couldn't find anything when doing a quick read through the man-pages. -- <Insert your favourite quote here.> Erik Trulsson [EMAIL PROTECTED] _______________________________________________ firstname.lastname@example.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "[EMAIL PROTECTED]"