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>hifive-unleashed-5.1
Song Liu
Shaohua Li
parent
03b047f45c
commit
39b99586b3
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@ -389,17 +389,30 @@ void r5c_check_cached_full_stripe(struct r5conf *conf)
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/*
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* Total log space (in sectors) needed to flush all data in cache
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*
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* Currently, writing-out phase automatically includes all pending writes
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* to the same sector. So the reclaim of each stripe takes up to
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* (conf->raid_disks + 1) pages of log space.
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* To avoid deadlock due to log space, it is necessary to reserve log
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* space to flush critical stripes (stripes that occupying log space near
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* last_checkpoint). This function helps check how much log space is
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* required to flush all cached stripes.
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*
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* To totally avoid deadlock due to log space, the code reserves
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* (conf->raid_disks + 1) pages for each stripe in cache, which is not
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* necessary in most cases.
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* To reduce log space requirements, two mechanisms are used to give cache
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* flush higher priorities:
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* 1. In handle_stripe_dirtying() and schedule_reconstruction(),
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* stripes ALREADY in journal can be flushed w/o pending writes;
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* 2. In r5l_write_stripe() and r5c_cache_data(), stripes NOT in journal
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* can be delayed (r5l_add_no_space_stripe).
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*
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* To improve this, we will need writing-out phase to be able to NOT include
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* pending writes, which will reduce the requirement to
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* (conf->max_degraded + 1) pages per stripe in cache.
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* In cache flush, the stripe goes through 1 and then 2. For a stripe that
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* already passed 1, flushing it requires at most (conf->max_degraded + 1)
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* pages of journal space. For stripes that has not passed 1, flushing it
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* requires (conf->raid_disks + 1) pages of journal space. There are at
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* most (conf->group_cnt + 1) stripe that passed 1. So total journal space
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* required to flush all cached stripes (in pages) is:
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*
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* (stripe_in_journal_count - group_cnt - 1) * (max_degraded + 1) +
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* (group_cnt + 1) * (raid_disks + 1)
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* or
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* (stripe_in_journal_count) * (max_degraded + 1) +
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* (group_cnt + 1) * (raid_disks - max_degraded)
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*/
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static sector_t r5c_log_required_to_flush_cache(struct r5conf *conf)
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{
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@ -408,8 +421,9 @@ static sector_t r5c_log_required_to_flush_cache(struct r5conf *conf)
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if (!r5c_is_writeback(log))
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return 0;
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return BLOCK_SECTORS * (conf->raid_disks + 1) *
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atomic_read(&log->stripe_in_journal_count);
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return BLOCK_SECTORS *
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((conf->max_degraded + 1) * atomic_read(&log->stripe_in_journal_count) +
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(conf->raid_disks - conf->max_degraded) * (conf->group_cnt + 1));
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}
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/*
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@ -2951,12 +2951,36 @@ sector_t raid5_compute_blocknr(struct stripe_head *sh, int i, int previous)
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* like to flush data in journal to RAID disks first, so complex rmw
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* is handled in the write patch (handle_stripe_dirtying).
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*
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* 2. when journal space is critical (R5C_LOG_CRITICAL=1)
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*
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* It is important to be able to flush all stripes in raid5-cache.
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* Therefore, we need reserve some space on the journal device for
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* these flushes. If flush operation includes pending writes to the
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* stripe, we need to reserve (conf->raid_disk + 1) pages per stripe
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* for the flush out. If we exclude these pending writes from flush
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* operation, we only need (conf->max_degraded + 1) pages per stripe.
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* Therefore, excluding pending writes in these cases enables more
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* efficient use of the journal device.
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*
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* Note: To make sure the stripe makes progress, we only delay
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* towrite for stripes with data already in journal (injournal > 0).
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* When LOG_CRITICAL, stripes with injournal == 0 will be sent to
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* no_space_stripes list.
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*
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*/
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static inline bool delay_towrite(struct r5dev *dev,
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struct stripe_head_state *s)
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static inline bool delay_towrite(struct r5conf *conf,
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struct r5dev *dev,
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struct stripe_head_state *s)
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{
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return !test_bit(R5_OVERWRITE, &dev->flags) &&
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!test_bit(R5_Insync, &dev->flags) && s->injournal;
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/* case 1 above */
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if (!test_bit(R5_OVERWRITE, &dev->flags) &&
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!test_bit(R5_Insync, &dev->flags) && s->injournal)
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return true;
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/* case 2 above */
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if (test_bit(R5C_LOG_CRITICAL, &conf->cache_state) &&
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s->injournal > 0)
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return true;
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return false;
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}
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static void
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@ -2979,7 +3003,7 @@ schedule_reconstruction(struct stripe_head *sh, struct stripe_head_state *s,
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for (i = disks; i--; ) {
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struct r5dev *dev = &sh->dev[i];
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if (dev->towrite && !delay_towrite(dev, s)) {
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if (dev->towrite && !delay_towrite(conf, dev, s)) {
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set_bit(R5_LOCKED, &dev->flags);
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set_bit(R5_Wantdrain, &dev->flags);
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if (!expand)
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@ -3731,7 +3755,7 @@ static int handle_stripe_dirtying(struct r5conf *conf,
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} else for (i = disks; i--; ) {
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/* would I have to read this buffer for read_modify_write */
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struct r5dev *dev = &sh->dev[i];
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if (((dev->towrite && !delay_towrite(dev, s)) ||
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if (((dev->towrite && !delay_towrite(conf, dev, s)) ||
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i == sh->pd_idx || i == sh->qd_idx ||
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test_bit(R5_InJournal, &dev->flags)) &&
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!test_bit(R5_LOCKED, &dev->flags) &&
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@ -3755,8 +3779,8 @@ static int handle_stripe_dirtying(struct r5conf *conf,
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}
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}
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pr_debug("for sector %llu, rmw=%d rcw=%d\n",
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(unsigned long long)sh->sector, rmw, rcw);
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pr_debug("for sector %llu state 0x%lx, rmw=%d rcw=%d\n",
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(unsigned long long)sh->sector, sh->state, rmw, rcw);
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set_bit(STRIPE_HANDLE, &sh->state);
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if ((rmw < rcw || (rmw == rcw && conf->rmw_level == PARITY_PREFER_RMW)) && rmw > 0) {
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/* prefer read-modify-write, but need to get some data */
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@ -3796,7 +3820,7 @@ static int handle_stripe_dirtying(struct r5conf *conf,
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for (i = disks; i--; ) {
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struct r5dev *dev = &sh->dev[i];
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if (((dev->towrite && !delay_towrite(dev, s)) ||
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if (((dev->towrite && !delay_towrite(conf, dev, s)) ||
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i == sh->pd_idx || i == sh->qd_idx ||
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test_bit(R5_InJournal, &dev->flags)) &&
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!test_bit(R5_LOCKED, &dev->flags) &&
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