Date: Mon, 16 Jun 2008 19:40:42 +0200 (CEST) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-questions@FreeBSD.ORG, wmoran@potentialtech.com, z.szalbot@lc-words.com Subject: Re: RAID 1 / disk error / Offline uncorrectable sectors Message-ID: <200806161740.m5GHegd6005955@lurza.secnetix.de> In-Reply-To: <20080616085534.ca7524f8.wmoran@potentialtech.com>
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Bill Moran wrote: > Zbigniew Szalbot wrote: > > [...] > > Jun 14 01:13:38 relay kernel: ad12: FAILURE - READ_DMA48 > > status=51<READY,DSC,ERROR> error=40<UNCORRECTABLE> LBA=374468863 > [...] > > Replace the hard drive. Every modern hard drive keeps extra space available > to "remap" bad sectors. This happens magically behind the scenes without > you ever knowing about it. Once you've hit "uncorrectable" errors, it means > your re-mappable sectors are used up, and that means the drive is on its > last legs. That's not completely true. When a disk drive encounters a bad sector during a read operation, it will remember the bad sector address, but it is unable to transparently remap the sector because it doesn't know that correct contents of the sector. So it has to report the unrecoverable error to the OS, even if there's still plenty of space for remapping sectors. Upon the next write operation to a sector marked as bad, the drive will finally remap it and write the data to a spare location. Therefore, getting "uncorrectable errors" does *not* mean that the drive has used up its spare sectors. You only need to overwrite the bad sectors (e.g. with dd(1))so the drive gets a chance to remap them. Of course, it might still be a good idea to replace the drive anyway. It depends on the cause of the bad sectors (mechanical or electrical). If you had a head crash (caused by mechanical impact or a media manufacturing error or whatever), it is possible that it caused debris within the drive which will cause further bad blocks. This can lead to a snowball effect that can really exhaust all spare sectors quickly. On the other hand, if the bad sectors where caused by a voltage spike, a power failure or similar, chances are that the drive is fine and you can continue to use it after making sure that the bad sectors are remapped (by overwriting them, see above). Finally, there is also the possibility that the problem is caused by a bug in the drive's firmware. If that's the case, I would be inclined to replace the drive with a different brand. However, I guess all drives have bugs ... the question is whether they affect you. Another question is whether it's possible at all to find out what caused the problem in the first place. Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd "What is this talk of 'release'? We do not make software 'releases'. Our software 'escapes', leaving a bloody trail of designers and quality assurance people in its wake."
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