After having repotted a Fedora Core 12 installation I was challenged with the apparently quite well-known warning message

/etc/cron.weekly/99-raid-check:

WARNING: mismatch_cnt is not 0 on /dev/md0

which came from Anacron into my inbox (The subject line starts with Anacron job ‘cron.weekly’). I remembered that I solved this problem on that Fedora server before, but could not remember how. Therefore I googled once again on this topic and found the following interesting links:

• http://www.mail-archive.com/gnhlug-discuss@mail.gnhlug.org/msg27222.html
• http://www.arrfab.net/blog/?p=199

Please note that those two points refer to CentOS, but apparently, the solution provided from Arrfab did also resolve my current problem. Please not just calling

echo repair >/sys/block/md0/md/sync_action

might not be enough to reset the mismatch_cnt as you can read at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=405919 but an additional

echo check >/sys/block/md0/md/sync_action

will be necessary. Afterwards you can cross check the mismatch_cnt with the command

cat /sys/block/md0/md/mismatch_cnt

I would like to state clearly that the raid1 array on which this happened does not contain any swap file.

FractializeR reports in his post that he calmed down, as soon as he read the memory-mapped file explanation. However, I cannot agree to his assessment, because up to that point to where the check run first time, I – for sure – did not execute any memory mapped operation (dd should not do so). So there still must be some different explanation for this out there. Thus, the post of Chris Siebenmann brings this unclearity to the core: unless you know from where it comes running the commands above remains risky.