Remake SW RAID1 from a new HDD and an old HDD with bad blocks
Question:
If you have a SW RAID1 and I just replaced, say, /dev/sda with a new HDD as the old one failed. Now, upon trying to recreate the RAID array you discover that the “good” HDD (/dev/sdb) has bad blocks which prevents mdadm from resyncing the array. While you could make backups, replace /dev/sdb as well and re-install the server completely, one is wondering if there is any way one could “trick” mdadm into resyncing the RAID array and then replace /dev/sdb with a new HDD. The badblocks should be located in an unused area of /dev/sdb which is only used when trying to recreate the RAID array, otherwise this data is lost to you.
Answer:
You should verify whether the affected blocks and underlying bad sectors on the disk are reallocated to “spare sectors” area? The bad sector should be reallocated when write operation fails. Verify it with smartctl:
[box]smartctl -a /dev/sdb | grep -i reallocated[/box]
The last column should contain a number of total reallocated sectors. If there is zero try to read the bad sector:
[box]hdparm –-read-sector XXXXXXXX /dev/sdb[/box]
It should return an I/O error otherwise I would recommend to skip next section.
The error means the sector was not reallocated yet. So you can try to reallocate it forcibly by writing it.
IMPORTANT: Remember that any data stored in this sector will be lost after this step !!!:
[box]hdparm –-write-sector XXXXXXXX --yes-i-know-what-i-am-doing /dev/sdb[/box]
By the way, the sector number XXXXXXXX should be possible to obtain from kernel messages (dmesg command or from /var/log/messages). As you had bad blocks during resynchronization there should be some related messages similar to:
[box]... end_request: I/O error, dev sdb, sector 1261071601[/box]
Then, try to verify it with smartctl again. Does the counter increased? If so try to read it with hdparm. Now, it should read it without any error as it is supposed to be reallocated. Done.
Finally, you can continue with mdadm and with adding the disk to your degraded mirror.
Juergen Vigna
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