Steve Thompson <smt@...> writes:

CentOS 6.3, x86_64.

I have noticed when building a new software RAID-6 array on CentOS 6.3 that the mismatch_cnt grows monotonically while the array is building:

# cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md11 : active raid6 sdg[5] sdf[4] sde[3] sdd[2] sdc[1] sdb[0] 3904890880 blocks super 1.2 level 6, 512k chunk, algorithm 2 [6/6]

[UUUUUU]

   [==================>..]  resync = 90.2% (880765600/976222720) 

finish=44.6min speed=35653K/sec

# cat /sys/block/md11/md/mismatch_cnt 1439285488

The mismatch count grows until the assembly is complete, and then remains at its highest value. A subsequent check resets it to zero (immediately) and everything is fine thereafter. The device is not in use by any other system component. I have reproduced this on several different systems; it always happens with CentOS 6.3 and never with CentOS 5.x and earlier (in 5.x, mismatch_cnt always stays at zero while assembling). I am using whole drives in this example, but it's the same if I use partitions instead. The count, size and type of drives appears to have no bearing.

Perhaps just a curiosity, but I'm curious as to why it does this.

Steve

This problem is not unique to CentOS -- I can confirm this same behavior on Ubuntu 12.10

mdv@ubuntu2:/sys/block/md0/md$ mdadm -V mdadm - v3.2.5 - 18th May 2012 mdv@ubuntu2:/sys/block/md0/md$ cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10] md0 : active raid6 sdm1[4] sdl1[3] sdj1[2] sdg1[1] sdf1[0] 8790398976 blocks super 1.2 level 6, 2048k chunk, algorithm 2 [5/5] [UUUUU] [======>..............] resync = 30.0% (879840768/2930132992) finish=978.5min speed=34918K/sec bitmap: 246/350 pages [984KB], 4096KB chunk, file: /var/lib/raid/md0bitmap

md1 : active raid6 sda[2] sdi1[1] sde1[4] sdd1[0] sdc1[3] 5860534272 blocks level 6, 1024k chunk, algorithm 2 [5/5] [UUUUU] bitmap: 11/15 pages [44KB], 65536KB chunk

unused devices: <none> mdv@ubuntu2:/sys/block/md0/md$ cat mismatch_cnt 1322557904 mdv@ubuntu2:/sys/block/md0/md$ cat rd?/errors 0 0 0 0 0 mdv@ubuntu2:/sys/block/md0/md$

Drives have been burned in without errors (/sbin/badblocks -v -w -b 4096), passed short and long SMART tests, and no SMART errors have been logged. No ata error messages in the kernel log files.

I've built this array several times with the same results. After a build completes, running root@ubuntu2:/sys/devices/virtual/block/md0/md# echo "repair" > sync_action ... wait 18 hours root@ubuntu2:/sys/devices/virtual/block/md0/md# echo "check" > sync_action ... wait 18 more hours will result in a mismatch_cnt of 0.