[ILUG] ext3 strangeness on software raid

[Niall O Broin]

I have a box using kernel software RAID-1 on two SCSI disks. There are two 
RAIDs, mdo consisting of sda5 and sdb5 and md1, consisting of sda6 and sdb6.
There is an ext3 FS on each of md0 and md1. Lately I have experienced some 
filesystem corruption on md0 which puzzles me, and cause me no small concern. 
I'm not yet in a position to boot the box from a rescue CD but in the absence 
of that I have been having a look around and I saw this in /var/log/messages


Mar 26 12:53:13 rh9box kernel: EXT3 FS 2.4-0.9.19, 19 August 2002 on sd(8,5), 
internal journal
Mar 26 12:53:13 rh9box kernel: Adding Swap: 2152700k swap-space (priority -1)
Mar 26 12:53:13 rh9box kernel: kjournald starting.  Commit interval 5 seconds
Mar 26 12:53:13 rh9box kernel: EXT3 FS 2.4-0.9.19, 19 August 2002 on md(9,1), 
internal journal
Mar 26 12:53:13 rh9box kernel: EXT3-fs: mounted filesystem with ordered data 
mode.
Mar 26 12:53:13 rh9box kernel: EXT3-fs error (device sd(8,5)): 
ext3_free_blocks: bit already cleared for block 17
04643

and a few seconds later, more errors.

Mar 26 12:53:40 rh9box kernel: EXT3-fs error (device sd(8,5)): 
ext3_free_blocks: bit already cleared for block 3999498
Mar 26 12:53:40 rh9box kernel: EXT3-fs error (device sd(8,5)): 
ext3_free_blocks: bit already cleared for block 3999502
Mar 26 12:53:40 rh9box kernel: EXT3-fs error (device sd(8,5)): 
ext3_free_blocks: bit already cleared for block 3999499
Mar 26 12:53:40 rh9box kernel: EXT3-fs error (device sd(8,5)): 
ext3_free_blocks: bit already cleared for block 3999500


What's puzzling me here is the reference to EXT3-fs error (device sd(8,5))  as 
there isn't an EXT3-fs on sd(8,5) (sda5) but rather on md0. Now I know that 
the way Linux RAID1 is made, you can effectively see the filesystem on either 
of the components of the RAID, but how is the kernel seeing this? There is 
never an attempt to access that partition on its own but only as a component 
of the RAID. There's no reference in /var/log/messages to sdb5 or sd[ab]6. I 
see no equivalents to


Mar 26 12:53:13 rh9box kernel: EXT3 FS 2.4-0.9.19, 19 August 2002 on sd(8,5), 
internal journal

for any other sd  partition.   Any ideas?


-- 
Niall