sync_action repair not reading all sectors?

[David Greaves <david@dgreaves.com>]

I have a drive that has bad sectors. Lots of them.

smartctl shows
# 1  Short offline       Completed: read failure       20%       530
1953520877

A simple ddrescue to this part of the disk gets this:

Mar 16 10:41:28 elm kernel: [ 8643.123397] sd 3:0:0:0: [sdd] 1953525168 512-byte
hardware sectors (1000205 MB)
<snip<>51/40:00:f0:5c:70/00:00:74:00:00/e0 Emask 0x9 (media error)
Mar 16 10:41:29 elm kernel: [ 8644.190060] ata4.00: status: { DRDY ERR }
Mar 16 10:41:29 elm kernel: [ 8644.190099] ata4.00: error: { UNC }

and reports 30 or so errors.


mdstat tells me:
md0 : active raid5 sdd1[0] sdb1[2] sda1[1]
      1953519872 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]

So sdd1 is in there.

/dev/sdd1 is the full disk

Now this is an enterprise class disk so I thought re-writing the blocks would be
worthwhile as a first step. (It is being RMAed but if it succeeds then I'll stop
the array, mirror/replace the disk and start the array - less risky than a resync).

However (two runs of)
  echo repair > /sys/block/md0/md/sync_action
ran to completion without *any* errors being reported in syslog (or anywhere)

Is this expected? It suggests that it isn't reading the bad parts of sdd. It
certainly hasn't repaired it and I'm none the wiser...

kernel is 2.6.26-1-xen-686
mdadm v2.6.7.2


PS
This is an excellent place where I'd love to add in a new 'spare' disk, mirror
sdd to the new disk (apart from the bad sectors which should come from the
array) and then swap new for old.
Instead I'm going to have to go degraded and sync - risking a sector read
failure on one of the other drives and a restore from backup :(

-- 
"Don't worry, you'll be fine; I saw it work in a cartoon once..."