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md/raid1: fix read balance when a drive is write-mostly.

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When a drive is marked write-mostly it should only be the
target of reads if there is no other option.

This behaviour was broken by

commit 9dedf60313
    md/raid1: read balance chooses idlest disk for SSD

which causes a write-mostly device to be *preferred* is some cases.

Restore correct behaviour by checking and setting
best_dist_disk and best_pending_disk rather than best_disk.

We only need to test one of these as they are both changed
from -1 or >=0 at the same time.

As we leave min_pending and best_dist unchanged, any non-write-mostly
device will appear better than the write-mostly device.

Reported-by: Tomáš Hodek <tomas.hodek@volny.cz>
Reported-by: Dark Penguin <darkpenguin@yandex.ru>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: http://marc.info/?l=linux-raid&m=135982797322422
Fixes: 9dedf60313
Cc: stable@vger.kernel.org (3.6+)
This commit is contained in:
Tomáš Hodek 2015-02-23 11:00:38 +11:00 committed by NeilBrown
parent c517d838eb
commit d1901ef099
1 changed files with 3 additions and 2 deletions
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5
drivers/md/raid1.c
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@ -560,7 +560,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
if (test_bit(WriteMostly, &rdev->flags)) {
/* Don't balance among write-mostly, just
* use the first as a last resort */
if (best_disk < 0) {
if (best_dist_disk < 0) {
if (is_badblock(rdev, this_sector, sectors,
&first_bad, &bad_sectors)) {
if (first_bad < this_sector)
@ -569,7 +569,8 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
best_good_sectors = first_bad - this_sector;
} else
best_good_sectors = sectors;
best_disk = disk;
best_dist_disk = disk;
best_pending_disk = disk;
}
continue;
}