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authorSong Liu <songliubraving@fb.com>2017-01-24 14:08:23 -0800
committerShaohua Li <shli@fb.com>2017-02-13 09:17:52 -0800
commit39b99586b321a9cbd4fe9abb5ea9346d2c4ca9c6 (patch)
tree8c4d3a516260a209f968c3d1755575433401340e /drivers/md/raid5-cache.c
parent03b047f45c29dff02f913a0234ca0cc1ca51966f (diff)
md/r5cache: improve journal device efficiency
It is important to be able to flush all stripes in raid5-cache. Therefore, we need reserve some space on the journal device for these flushes. If flush operation includes pending writes to the stripe, we need to reserve (conf->raid_disk + 1) pages per stripe for the flush out. This reduces the efficiency of journal space. If we exclude these pending writes from flush operation, we only need (conf->max_degraded + 1) pages per stripe. With this patch, when log space is critical (R5C_LOG_CRITICAL=1), pending writes will be excluded from stripe flush out. Therefore, we can reduce reserved space for flush out and thus improve journal device efficiency. Signed-off-by: Song Liu <songliubraving@fb.com> Signed-off-by: Shaohua Li <shli@fb.com>
Diffstat (limited to 'drivers/md/raid5-cache.c')
-rw-r--r--drivers/md/raid5-cache.c36
1 files changed, 25 insertions, 11 deletions
diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 76c0e5063f1b..caae85331a02 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -389,17 +389,30 @@ void r5c_check_cached_full_stripe(struct r5conf *conf)
/*
* Total log space (in sectors) needed to flush all data in cache
*
- * Currently, writing-out phase automatically includes all pending writes
- * to the same sector. So the reclaim of each stripe takes up to
- * (conf->raid_disks + 1) pages of log space.
+ * To avoid deadlock due to log space, it is necessary to reserve log
+ * space to flush critical stripes (stripes that occupying log space near
+ * last_checkpoint). This function helps check how much log space is
+ * required to flush all cached stripes.
*
- * To totally avoid deadlock due to log space, the code reserves
- * (conf->raid_disks + 1) pages for each stripe in cache, which is not
- * necessary in most cases.
+ * To reduce log space requirements, two mechanisms are used to give cache
+ * flush higher priorities:
+ * 1. In handle_stripe_dirtying() and schedule_reconstruction(),
+ * stripes ALREADY in journal can be flushed w/o pending writes;
+ * 2. In r5l_write_stripe() and r5c_cache_data(), stripes NOT in journal
+ * can be delayed (r5l_add_no_space_stripe).
*
- * To improve this, we will need writing-out phase to be able to NOT include
- * pending writes, which will reduce the requirement to
- * (conf->max_degraded + 1) pages per stripe in cache.
+ * In cache flush, the stripe goes through 1 and then 2. For a stripe that
+ * already passed 1, flushing it requires at most (conf->max_degraded + 1)
+ * pages of journal space. For stripes that has not passed 1, flushing it
+ * requires (conf->raid_disks + 1) pages of journal space. There are at
+ * most (conf->group_cnt + 1) stripe that passed 1. So total journal space
+ * required to flush all cached stripes (in pages) is:
+ *
+ * (stripe_in_journal_count - group_cnt - 1) * (max_degraded + 1) +
+ * (group_cnt + 1) * (raid_disks + 1)
+ * or
+ * (stripe_in_journal_count) * (max_degraded + 1) +
+ * (group_cnt + 1) * (raid_disks - max_degraded)
*/
static sector_t r5c_log_required_to_flush_cache(struct r5conf *conf)
{
@@ -408,8 +421,9 @@ static sector_t r5c_log_required_to_flush_cache(struct r5conf *conf)
if (!r5c_is_writeback(log))
return 0;
- return BLOCK_SECTORS * (conf->raid_disks + 1) *
- atomic_read(&log->stripe_in_journal_count);
+ return BLOCK_SECTORS *
+ ((conf->max_degraded + 1) * atomic_read(&log->stripe_in_journal_count) +
+ (conf->raid_disks - conf->max_degraded) * (conf->group_cnt + 1));
}
/*