Date: Fri, 11 Jul 2003 18:23:33 -0700 (PDT) From: Dan Strick <strick@covad.net> To: questions@freebsd.org Cc: dan@ice.nodomain Subject: Re: Advise on data recovery from failed drive Message-ID: <200307120123.h6C1NXBI001008@ice.nodomain>
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On Friday July 12, 2003 (PST) Per olof Ljungmark wrote: >>>>>>>>>>>>>>>>>>> > I've had a drive crash where the spindle motor bearings overheated and > got stuck. > Using mild violence I now have the drive spinning again and need to do > some data recovery. > > It has to be something that is able to handle read errors without stopping, > I am thinking dd, any other suggestions? >>>>>>>>>>>>>>>>>>>>> I have an ancient program that I once wrote to make copies of disk drives with bad sectors. It normally reads/writes in large units, but goes back and rereads a section of the input disk one 512 byte sector at a time if it gets a read error. It will retry a single sector read several times before giving up. Unreadable sectors are "assumed" to be zero. This program was intended to be used with raw disk devices (drivers) doing unbuffered "physical" I/O with simple error recovery procedures (perhaps a limited number of read retries). I have not "ported" it to FreeBSD or even looked at it in many years, but it ought to work ok as is. The traditional dd program has a few problems if you use it with the "conv=noerror" option to copy sick disks. One problem is that you must specify "bs=512" (i.e. copy only one sector at a time) to avoid losing good disk sectors adjacent to a bad disk sector. This makes for a slow copy. Another problem is that is that if you tell dd to ignore input errors, it skips the bad blocks on input but not on output so that after a read error blocks are copied to a wrong disk address. I just did a man on the FreeBSD 4.8-RELEASE dd command and discovered a new option that avoids this problem. Specify "conv=noerror,sync" if you use the dd command. Dan Strick
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