Maarten van den Berg wrote:
cables and such. So my obvious question is: Is this step (mkraid --force with one of the offline disks defined as failed-disk) destructive, or could I (theoretically) experiment endlessly with the order in which the disks are defined in /etc/raidtab before I decide to mount it read-write and raidhotadd a fresh disk ?I had to do this about a year ago, on account of a bad IDE controller, and was successful on the first try. I recall from the HOWTO that the 'mkraid --force' command IS destructive, and it WILL lose the array if you get it wrong.
My incident involved data which would have been a bear to restore from backup, so I didn't take any chances. Prior to the 'mkraid --force' step, I labelled each physical disk with it's number in the array, and then copied each disk to an identical disk using the 'dd' command [somthing like 'dd if=/dev/sda of=/dev/sdb bs=8192' where /dev/sda is the original and /dev/sdb is a handy blank disk, repeated for each original disk in the array]. Of course, this required five extra hard disks, but I had them lying around anyway. And that's the foolproof method. If you're really paranoid (you don't have a backup of the failed array), you might want to also do an 'md5sum /dev/sdb; md5sum /dev/sda' and compare the two hash values to ensure that each copy is faithful--re-do the copy that doesn't pass the hash check. Since you're juggling so many hard drives, make sure you label all the disks--scotch-taped sticky notes on the cover, with the array #s written in felt pen worked for me.
Then, you can run off and do whatever with the originals, and you'll always have something to go back to if you utterly wipe out your array while trying to restore it. If that happens, you just take the clobbered disks, and re-do the 'dd' command to write the copy back to the originals. Hash if necessary, and go to town again. Repeat until you either successfully restore the array, or you swear off computers and enroll in florists' school.
Keep in mind that this can add a LOT of time to a restore operation. With Western Digital WD200 IDE disks (7200 RPM, 18.6 GB), I can move about 30-22 MB/sec (beginning-end) on direct disk-to-disk copies like this, with an average around 25 MB/sec. That's ~1.5 GB/min, so you can estimate your own ETAs. Hashing will take a similar amount of time if you use 'md5sum' on a reasonably fast machine (P4 1.8+).
-R
- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
