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I have a server that peforms a resync for its software raid in undefined intervals. This results in massive load and I am forced to stop all services until the resync is complete (20+ hours). What could be the reason for an automatic resync?

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In some cases, cron. On RedHat-derived boxes, check for the existence of a file like /etc/cron.weekly/99-raid-check; on other distros, it can be a file in the /etc/cron.d/ directory.

Edit following your question: a check is not the same as a resync. A check compares the two halves of a RAID-1 array and updates /sys/block/mdX/md/mismatch_cnt with the current count of differences between the two halves. But it requires reading every single bit of both halves of the mirror, and is thus hugely IO-intensive (well, input-intensive).

If you find the mismatch count is non-zero, you can then choose to force a resync. Your system's script may do that for you as part of the service, I can't comment, but there is a difference between a check and a resync. Both are hugely IO-intensive, though, and result in the sort of performance issues you describe.

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    It's a debian box. I found /etc/cron.d/mdadm, In it stands 57 0 * * 0 root [ -x /usr/share/mdadm/checkarray ] && [ $(date +\%d) -le 7 ] && /usr/share/mdadm/checkarray --cron --all --quiet Does a check automatically result in a resync?
    – Philip
    Apr 4 '11 at 17:21
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    @Philip, see the comment in that file. It runs a resync the first sudany of every month. That file is a script so you could easily look at it. Also see its configuration file /etc/default/mdadm.
    – Zoredache
    Apr 4 '11 at 17:27

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