mdadm - Currently unreadable sectors on RAID 5 linux drive

itemprop="text">

I have every 30 minutes smartd
messages on
/var/log/messages:

smartd[3588]:
Device: /dev/sdc, 176 Currently unreadable (pending)
sectors

This drive (sdc) is part of RAID 5
configured with mdadm.
Mdadm monitor tells RAID is ok but i want to know if i
need to change the drive or not. Also if its neccesary to mark as bad this sectors or OS
already did it.
If i need to change the drive, how can i chose the replacement
one? I can´t find the number of blocks in hard drive specifications so if i chose one
with less blocks than original, i will be in
trouble.
Thanks.

itemprop="text">

class="normal">Answer

Yes,
change the drive.

Unreadable
(pending) sectors are sector whose contents could not be read. On a normal non-RAID
situation that would result in either a read error, or a long delay while the drive
attempts to read the sector again and again until it succeeds (or until it eventually
gives up).

With RAID two things are
happening:

1. Your disk is
   probably configured with a short TLER value. Thus is will give up its attempts to read
   that sector within a reasonable time. (Thus preventing long
   hangs).


2. Your RAID array notices the failure and reads the
   data from another disk. This is the advantage of RAID 5; you have a spare
   copy.

What you want to do
is:

1. Check your
   backups. You should not need them if all goes
   well.


2. Fetch a replacement disk of equal or
   larger size. You can check the size with smartctl -a /dev/sdc.
   Do not assume all drives of size X have the same capacity. Manufacturers like round
   numbers; one 500GB drive might well be smaller than another 500 GB
   drive.


3. Bring the disk with problems off-line.
   (mdadm --manage --remove /dev/mdX
/dev/sdc)


4. Replace the disk with new hardware
   and let the array rebuild itself. (mdadm --add /dev/mdX
/dev/sdc)

If
you used large disks then this will take a lot of time. Sometimes it is faster to just
rebuild the RAID array from scratch and restore from backups. (TEST those backups
first!)

While the RAID is rebuilding
you have no redundancy. Thus is another disk fails (e.g. due to the stress of
rebuilding) then you have a problem. This sometimes happens with large disks (long
rebuild times) and batches of drives from the same date.