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md: improve the is_mddev_idle test
During a 'resync' or similar activity, md checks if the devices in the array are otherwise active and winds back resync activity when they are. This test in done in is_mddev_idle, and it is somewhat fragile - it sometimes thinks there is non-sync io when there isn't. The test compares the total sectors of io (disk_stat_read) with the sectors of resync io (disk->sync_io). This has problems because total sectors gets updated when a request completes, while resync io gets updated when the request is submitted. The time difference can cause large differenced between the two which do not actually imply non-resync activity. The test currently allows for some fuzz (+/- 4096) but there are some cases when it is not enough. The test currently looks for any (non-fuzz) difference, either positive or negative. This clearly is not needed. Any non-sync activity will cause the total sectors to grow faster than the sync_io count (never slower) so we only need to look for a positive differences. If we do this then the amount of in-flight sync io will never cause the appearance of non-sync IO. Once enough non-sync IO to worry about starts happening, resync will be slowed down and the measurements will thus be more precise (as there is less in-flight) and control of resync will still be suitably responsive. Signed-off-by: Neil Brown <neilb@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -5103,7 +5103,7 @@ static int is_mddev_idle(mddev_t *mddev)
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*
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* Note: the following is an unsigned comparison.
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*/
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if ((curr_events - rdev->last_events + 4096) > 8192) {
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if ((long)curr_events - (long)rdev->last_events > 4096) {
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rdev->last_events = curr_events;
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idle = 0;
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}
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