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md/r5cache: State machine for raid5-cache write back mode

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This patch adds state machine for raid5-cache. With log device, the
raid456 array could operate in two different modes (r5c_journal_mode):
  - write-back (R5C_MODE_WRITE_BACK)
  - write-through (R5C_MODE_WRITE_THROUGH)

Existing code of raid5-cache only has write-through mode. For write-back
cache, it is necessary to extend the state machine.

With write-back cache, every stripe could operate in two different
phases:
  - caching
  - writing-out

In caching phase, the stripe handles writes as:
  - write to journal
  - return IO

In writing-out phase, the stripe behaviors as a stripe in write through
mode R5C_MODE_WRITE_THROUGH.

STRIPE_R5C_CACHING is added to sh->state to differentiate caching and
writing-out phase.

Please note: this is a "no-op" patch for raid5-cache write-through
mode.

The following detailed explanation is copied from the raid5-cache.c:

/*
 * raid5 cache state machine
 *
 * With rhe RAID cache, each stripe works in two phases:
 *      - caching phase
 *      - writing-out phase
 *
 * These two phases are controlled by bit STRIPE_R5C_CACHING:
 *   if STRIPE_R5C_CACHING == 0, the stripe is in writing-out phase
 *   if STRIPE_R5C_CACHING == 1, the stripe is in caching phase
 *
 * When there is no journal, or the journal is in write-through mode,
 * the stripe is always in writing-out phase.
 *
 * For write-back journal, the stripe is sent to caching phase on write
 * (r5c_handle_stripe_dirtying). r5c_make_stripe_write_out() kicks off
 * the write-out phase by clearing STRIPE_R5C_CACHING.
 *
 * Stripes in caching phase do not write the raid disks. Instead, all
 * writes are committed from the log device. Therefore, a stripe in
 * caching phase handles writes as:
 *      - write to log device
 *      - return IO
 *
 * Stripes in writing-out phase handle writes as:
 *      - calculate parity
 *      - write pending data and parity to journal
 *      - write data and parity to raid disks
 *      - return IO for pending writes
 */

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
This commit is contained in:
Song Liu
2016-11-17 15:24:38 -08:00
committed by Shaohua Li
parent 937621c36e
commit 2ded370373
3 changed files with 211 additions and 8 deletions
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45
drivers/md/raid5.c
View File

@@ -4107,6 +4107,9 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
if (rdev && !test_bit(Faulty, &rdev->flags))
do_recovery = 1;
}
if (test_bit(R5_InJournal, &dev->flags))
s->injournal++;
}
if (test_bit(STRIPE_SYNCING, &sh->state)) {
/* If there is a failed device being replaced,
@@ -4386,14 +4389,47 @@ static void handle_stripe(struct stripe_head *sh)
|| s.expanding)
handle_stripe_fill(sh, &s, disks);
/* Now to consider new write requests and what else, if anything
* should be read. We do not handle new writes when:
/*
* When the stripe finishes full journal write cycle (write to journal
* and raid disk), this is the clean up procedure so it is ready for
* next operation.
*/
r5c_finish_stripe_write_out(conf, sh, &s);
/*
* Now to consider new write requests, cache write back and what else,
* if anything should be read. We do not handle new writes when:
* 1/ A 'write' operation (copy+xor) is already in flight.
* 2/ A 'check' operation is in flight, as it may clobber the parity
* block.
* 3/ A r5c cache log write is in flight.
*/
if (s.to_write && !sh->reconstruct_state && !sh->check_state)
handle_stripe_dirtying(conf, sh, &s, disks);
if (!sh->reconstruct_state && !sh->check_state && !sh->log_io) {
if (!r5c_is_writeback(conf->log)) {
if (s.to_write)
handle_stripe_dirtying(conf, sh, &s, disks);
} else { /* write back cache */
int ret = 0;
/* First, try handle writes in caching phase */
if (s.to_write)
ret = r5c_try_caching_write(conf, sh, &s,
disks);
/*
* If caching phase failed: ret == -EAGAIN
* OR
* stripe under reclaim: !caching && injournal
*
* fall back to handle_stripe_dirtying()
*/
if (ret == -EAGAIN ||
/* stripe under reclaim: !caching && injournal */
(!test_bit(STRIPE_R5C_CACHING, &sh->state) &&
s.injournal > 0))
handle_stripe_dirtying(conf, sh, &s, disks);
}
}
/* maybe we need to check and possibly fix the parity for this stripe
* Any reads will already have been scheduled, so we just see if enough
@@ -5110,6 +5146,7 @@ static void raid5_make_request(struct mddev *mddev, struct bio * bi)
* data on failed drives.
*/
if (rw == READ && mddev->degraded == 0 &&
!r5c_is_writeback(conf->log) &&
mddev->reshape_position == MaxSector) {
bi = chunk_aligned_read(mddev, bi);
if (!bi)