| Message ID | 20210519121317.585244-8-david@fromorbit.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | xfs: CIL and log optimisations | expand |
On Wed, May 19, 2021 at 10:12:45PM +1000, Dave Chinner wrote: > From: Dave Chinner <dchinner@redhat.com> > > Currently every journal IO is issued as REQ_PREFLUSH | REQ_FUA to > guarantee the ordering requirements the journal has w.r.t. metadata > writeback. THe two ordering constraints are: > > 1. we cannot overwrite metadata in the journal until we guarantee > that the dirty metadata has been written back in place and is > stable. > > 2. we cannot write back dirty metadata until it has been written to > the journal and guaranteed to be stable (and hence recoverable) in > the journal. > > The ordering guarantees of #1 are provided by REQ_PREFLUSH. This > causes the journal IO to issue a cache flush and wait for it to > complete before issuing the write IO to the journal. Hence all > completed metadata IO is guaranteed to be stable before the journal > overwrites the old metadata. > > The ordering guarantees of #2 are provided by the REQ_FUA, which > ensures the journal writes do not complete until they are on stable > storage. Hence by the time the last journal IO in a checkpoint > completes, we know that the entire checkpoint is on stable storage > and we can unpin the dirty metadata and allow it to be written back. > > This is the mechanism by which ordering was first implemented in XFS > way back in 2002 by commit 95d97c36e5155075ba2eb22b17562cfcc53fcf96 > ("Add support for drive write cache flushing") in the xfs-archive > tree. > > A lot has changed since then, most notably we now use delayed > logging to checkpoint the filesystem to the journal rather than > write each individual transaction to the journal. Cache flushes on > journal IO are necessary when individual transactions are wholly > contained within a single iclog. However, CIL checkpoints are single > transactions that typically span hundreds to thousands of individual > journal writes, and so the requirements for device cache flushing > have changed. > > That is, the ordering rules I state above apply to ordering of > atomic transactions recorded in the journal, not to the journal IO > itself. Hence we need to ensure metadata is stable before we start > writing a new transaction to the journal (guarantee #1), and we need > to ensure the entire transaction is stable in the journal before we > start metadata writeback (guarantee #2). > > Hence we only need a REQ_PREFLUSH on the journal IO that starts a > new journal transaction to provide #1, and it is not on any other > journal IO done within the context of that journal transaction. > > The CIL checkpoint already issues a cache flush before it starts > writing to the log, so we no longer need the iclog IO to issue a > REQ_REFLUSH for us. Hence if XLOG_START_TRANS is passed > to xlog_write(), we no longer need to mark the first iclog in > the log write with REQ_PREFLUSH for this case. As an added bonus, > this ordering mechanism works for both internal and external logs, > meaning we can remove the explicit data device cache flushes from > the iclog write code when using external logs. > > Given the new ordering semantics of commit records for the CIL, we > need iclogs containing commit records to issue a REQ_PREFLUSH. We > also require unmount records to do this. Hence for both > XLOG_COMMIT_TRANS and XLOG_UNMOUNT_TRANS xlog_write() calls we need > to mark the first iclog being written with REQ_PREFLUSH. > > For both commit records and unmount records, we also want them > immediately on stable storage, so we want to also mark the iclogs > that contain these records to be marked REQ_FUA. That means if a > record is split across multiple iclogs, they are all marked REQ_FUA > and not just the last one so that when the transaction is completed > all the parts of the record are on stable storage. > > And for external logs, unmount records need a pre-write data device > cache flush similar to the CIL checkpoint cache pre-flush as the > internal iclog write code does not do this implicitly anymore. > > As an optimisation, when the commit record lands in the same iclog > as the journal transaction starts, we don't need to wait for > anything and can simply use REQ_FUA to provide guarantee #2. This > means that for fsync() heavy workloads, the cache flush behaviour is > completely unchanged and there is no degradation in performance as a > result of optimise the multi-IO transaction case. > > The most notable sign that there is less IO latency on my test > machine (nvme SSDs) is that the "noiclogs" rate has dropped > substantially. This metric indicates that the CIL push is blocking > in xlog_get_iclog_space() waiting for iclog IO completion to occur. > With 8 iclogs of 256kB, the rate is appoximately 1 noiclog event to > every 4 iclog writes. IOWs, every 4th call to xlog_get_iclog_space() > is blocking waiting for log IO. With the changes in this patch, this > drops to 1 noiclog event for every 100 iclog writes. Hence it is > clear that log IO is completing much faster than it was previously, > but it is also clear that for large iclog sizes, this isn't the > performance limiting factor on this hardware. > > With smaller iclogs (32kB), however, there is a sustantial s/sustantial/substantial/ > difference. With the cache flush modifications, the journal is now > running at over 4000 write IOPS, and the journal throughput is > largely identical to the 256kB iclogs and the noiclog event rate > stays low at about 1:50 iclog writes. The existing code tops out at > about 2500 IOPS as the number of cache flushes dominate performance > and latency. The noiclog event rate is about 1:4, and the > performance variance is quite large as the journal throughput can > fall to less than half the peak sustained rate when the cache flush > rate prevents metadata writeback from keeping up and the log runs > out of space and throttles reservations. > > As a result: > > logbsize fsmark create rate rm -rf > before 32kb 152851+/-5.3e+04 5m28s > patched 32kb 221533+/-1.1e+04 5m24s > > before 256kb 220239+/-6.2e+03 4m58s > patched 256kb 228286+/-9.2e+03 5m06s > > The rm -rf times are included because I ran them, but the > differences are largely noise. This workload is largely metadata > read IO latency bound and the changes to the journal cache flushing > doesn't really make any noticable difference to behaviour apart from > a reduction in noiclog events from background CIL pushing. > > Signed-off-by: Dave Chinner <dchinner@redhat.com> > Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com> With that fixed, Reviewed-by: Darrick J. Wong <djwong@kernel.org> --D > --- > fs/xfs/xfs_log.c | 66 +++++++++++++++---------------------------- > fs/xfs/xfs_log.h | 1 - > fs/xfs/xfs_log_cil.c | 18 +++++++++--- > fs/xfs/xfs_log_priv.h | 6 ++++ > 4 files changed, 43 insertions(+), 48 deletions(-) > > diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c > index 87870867d9fb..b6145e4cb7bc 100644 > --- a/fs/xfs/xfs_log.c > +++ b/fs/xfs/xfs_log.c > @@ -513,7 +513,7 @@ __xlog_state_release_iclog( > * Flush iclog to disk if this is the last reference to the given iclog and the > * it is in the WANT_SYNC state. > */ > -static int > +int > xlog_state_release_iclog( > struct xlog *log, > struct xlog_in_core *iclog) > @@ -533,23 +533,6 @@ xlog_state_release_iclog( > return 0; > } > > -void > -xfs_log_release_iclog( > - struct xlog_in_core *iclog) > -{ > - struct xlog *log = iclog->ic_log; > - bool sync = false; > - > - if (atomic_dec_and_lock(&iclog->ic_refcnt, &log->l_icloglock)) { > - if (iclog->ic_state != XLOG_STATE_IOERROR) > - sync = __xlog_state_release_iclog(log, iclog); > - spin_unlock(&log->l_icloglock); > - } > - > - if (sync) > - xlog_sync(log, iclog); > -} > - > /* > * Mount a log filesystem > * > @@ -837,6 +820,14 @@ xlog_write_unmount_record( > > /* account for space used by record data */ > ticket->t_curr_res -= sizeof(ulf); > + > + /* > + * For external log devices, we need to flush the data device cache > + * first to ensure all metadata writeback is on stable storage before we > + * stamp the tail LSN into the unmount record. > + */ > + if (log->l_targ != log->l_mp->m_ddev_targp) > + blkdev_issue_flush(log->l_targ->bt_bdev); > return xlog_write(log, &vec, ticket, NULL, NULL, XLOG_UNMOUNT_TRANS); > } > > @@ -874,6 +865,11 @@ xlog_unmount_write( > else > ASSERT(iclog->ic_state == XLOG_STATE_WANT_SYNC || > iclog->ic_state == XLOG_STATE_IOERROR); > + /* > + * Ensure the journal is fully flushed and on stable storage once the > + * iclog containing the unmount record is written. > + */ > + iclog->ic_flags |= (XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA); > error = xlog_state_release_iclog(log, iclog); > xlog_wait_on_iclog(iclog); > > @@ -1755,8 +1751,7 @@ xlog_write_iclog( > struct xlog *log, > struct xlog_in_core *iclog, > uint64_t bno, > - unsigned int count, > - bool need_flush) > + unsigned int count) > { > ASSERT(bno < log->l_logBBsize); > > @@ -1794,10 +1789,12 @@ xlog_write_iclog( > * writeback throttle from throttling log writes behind background > * metadata writeback and causing priority inversions. > */ > - iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | > - REQ_IDLE | REQ_FUA; > - if (need_flush) > + iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | REQ_IDLE; > + if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH) > iclog->ic_bio.bi_opf |= REQ_PREFLUSH; > + if (iclog->ic_flags & XLOG_ICL_NEED_FUA) > + iclog->ic_bio.bi_opf |= REQ_FUA; > + iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA); > > if (xlog_map_iclog_data(&iclog->ic_bio, iclog->ic_data, count)) { > xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR); > @@ -1900,7 +1897,6 @@ xlog_sync( > unsigned int roundoff; /* roundoff to BB or stripe */ > uint64_t bno; > unsigned int size; > - bool need_flush = true, split = false; > > ASSERT(atomic_read(&iclog->ic_refcnt) == 0); > > @@ -1925,10 +1921,8 @@ xlog_sync( > bno = BLOCK_LSN(be64_to_cpu(iclog->ic_header.h_lsn)); > > /* Do we need to split this write into 2 parts? */ > - if (bno + BTOBB(count) > log->l_logBBsize) { > + if (bno + BTOBB(count) > log->l_logBBsize) > xlog_split_iclog(log, &iclog->ic_header, bno, count); > - split = true; > - } > > /* calculcate the checksum */ > iclog->ic_header.h_crc = xlog_cksum(log, &iclog->ic_header, > @@ -1949,22 +1943,8 @@ xlog_sync( > be64_to_cpu(iclog->ic_header.h_lsn)); > } > #endif > - > - /* > - * Flush the data device before flushing the log to make sure all meta > - * data written back from the AIL actually made it to disk before > - * stamping the new log tail LSN into the log buffer. For an external > - * log we need to issue the flush explicitly, and unfortunately > - * synchronously here; for an internal log we can simply use the block > - * layer state machine for preflushes. > - */ > - if (log->l_targ != log->l_mp->m_ddev_targp || split) { > - blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev); > - need_flush = false; > - } > - > xlog_verify_iclog(log, iclog, count); > - xlog_write_iclog(log, iclog, bno, count, need_flush); > + xlog_write_iclog(log, iclog, bno, count); > } > > /* > @@ -2418,7 +2398,7 @@ xlog_write( > ASSERT(log_offset <= iclog->ic_size - 1); > ptr = iclog->ic_datap + log_offset; > > - /* start_lsn is the first lsn written to. That's all we need. */ > + /* Start_lsn is the first lsn written to. */ > if (start_lsn && !*start_lsn) > *start_lsn = be64_to_cpu(iclog->ic_header.h_lsn); > > diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h > index 044e02cb8921..99f9d6ed9598 100644 > --- a/fs/xfs/xfs_log.h > +++ b/fs/xfs/xfs_log.h > @@ -117,7 +117,6 @@ void xfs_log_mount_cancel(struct xfs_mount *); > xfs_lsn_t xlog_assign_tail_lsn(struct xfs_mount *mp); > xfs_lsn_t xlog_assign_tail_lsn_locked(struct xfs_mount *mp); > void xfs_log_space_wake(struct xfs_mount *mp); > -void xfs_log_release_iclog(struct xlog_in_core *iclog); > int xfs_log_reserve(struct xfs_mount *mp, > int length, > int count, > diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c > index 172bb3551d6b..9d2fa8464289 100644 > --- a/fs/xfs/xfs_log_cil.c > +++ b/fs/xfs/xfs_log_cil.c > @@ -890,15 +890,25 @@ xlog_cil_push_work( > > /* > * If the checkpoint spans multiple iclogs, wait for all previous > - * iclogs to complete before we submit the commit_iclog. > + * iclogs to complete before we submit the commit_iclog. In this case, > + * the commit_iclog write needs to issue a pre-flush so that the > + * ordering is correctly preserved down to stable storage. > */ > + spin_lock(&log->l_icloglock); > if (ctx->start_lsn != commit_lsn) { > - spin_lock(&log->l_icloglock); > xlog_wait_on_iclog(commit_iclog->ic_prev); > + spin_lock(&log->l_icloglock); > + commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH; > } > > - /* release the hounds! */ > - xfs_log_release_iclog(commit_iclog); > + /* > + * The commit iclog must be written to stable storage to guarantee > + * journal IO vs metadata writeback IO is correctly ordered on stable > + * storage. > + */ > + commit_iclog->ic_flags |= XLOG_ICL_NEED_FUA; > + xlog_state_release_iclog(log, commit_iclog); > + spin_unlock(&log->l_icloglock); > return; > > out_skip: > diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h > index 56e1942c47df..2203ccecafb6 100644 > --- a/fs/xfs/xfs_log_priv.h > +++ b/fs/xfs/xfs_log_priv.h > @@ -133,6 +133,9 @@ enum xlog_iclog_state { > > #define XLOG_COVER_OPS 5 > > +#define XLOG_ICL_NEED_FLUSH (1 << 0) /* iclog needs REQ_PREFLUSH */ > +#define XLOG_ICL_NEED_FUA (1 << 1) /* iclog needs REQ_FUA */ > + > /* Ticket reservation region accounting */ > #define XLOG_TIC_LEN_MAX 15 > > @@ -201,6 +204,7 @@ typedef struct xlog_in_core { > u32 ic_size; > u32 ic_offset; > enum xlog_iclog_state ic_state; > + unsigned int ic_flags; > char *ic_datap; /* pointer to iclog data */ > > /* Callback structures need their own cacheline */ > @@ -486,6 +490,8 @@ int xlog_commit_record(struct xlog *log, struct xlog_ticket *ticket, > void xfs_log_ticket_ungrant(struct xlog *log, struct xlog_ticket *ticket); > void xfs_log_ticket_regrant(struct xlog *log, struct xlog_ticket *ticket); > > +int xlog_state_release_iclog(struct xlog *log, struct xlog_in_core *iclog); > + > /* > * When we crack an atomic LSN, we sample it first so that the value will not > * change while we are cracking it into the component values. This means we > -- > 2.31.1 >
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c index 87870867d9fb..b6145e4cb7bc 100644 --- a/fs/xfs/xfs_log.c +++ b/fs/xfs/xfs_log.c @@ -513,7 +513,7 @@ __xlog_state_release_iclog( * Flush iclog to disk if this is the last reference to the given iclog and the * it is in the WANT_SYNC state. */ -static int +int xlog_state_release_iclog( struct xlog *log, struct xlog_in_core *iclog) @@ -533,23 +533,6 @@ xlog_state_release_iclog( return 0; } -void -xfs_log_release_iclog( - struct xlog_in_core *iclog) -{ - struct xlog *log = iclog->ic_log; - bool sync = false; - - if (atomic_dec_and_lock(&iclog->ic_refcnt, &log->l_icloglock)) { - if (iclog->ic_state != XLOG_STATE_IOERROR) - sync = __xlog_state_release_iclog(log, iclog); - spin_unlock(&log->l_icloglock); - } - - if (sync) - xlog_sync(log, iclog); -} - /* * Mount a log filesystem * @@ -837,6 +820,14 @@ xlog_write_unmount_record( /* account for space used by record data */ ticket->t_curr_res -= sizeof(ulf); + + /* + * For external log devices, we need to flush the data device cache + * first to ensure all metadata writeback is on stable storage before we + * stamp the tail LSN into the unmount record. + */ + if (log->l_targ != log->l_mp->m_ddev_targp) + blkdev_issue_flush(log->l_targ->bt_bdev); return xlog_write(log, &vec, ticket, NULL, NULL, XLOG_UNMOUNT_TRANS); } @@ -874,6 +865,11 @@ xlog_unmount_write( else ASSERT(iclog->ic_state == XLOG_STATE_WANT_SYNC || iclog->ic_state == XLOG_STATE_IOERROR); + /* + * Ensure the journal is fully flushed and on stable storage once the + * iclog containing the unmount record is written. + */ + iclog->ic_flags |= (XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA); error = xlog_state_release_iclog(log, iclog); xlog_wait_on_iclog(iclog); @@ -1755,8 +1751,7 @@ xlog_write_iclog( struct xlog *log, struct xlog_in_core *iclog, uint64_t bno, - unsigned int count, - bool need_flush) + unsigned int count) { ASSERT(bno < log->l_logBBsize); @@ -1794,10 +1789,12 @@ xlog_write_iclog( * writeback throttle from throttling log writes behind background * metadata writeback and causing priority inversions. */ - iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | - REQ_IDLE | REQ_FUA; - if (need_flush) + iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | REQ_IDLE; + if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH) iclog->ic_bio.bi_opf |= REQ_PREFLUSH; + if (iclog->ic_flags & XLOG_ICL_NEED_FUA) + iclog->ic_bio.bi_opf |= REQ_FUA; + iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA); if (xlog_map_iclog_data(&iclog->ic_bio, iclog->ic_data, count)) { xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR); @@ -1900,7 +1897,6 @@ xlog_sync( unsigned int roundoff; /* roundoff to BB or stripe */ uint64_t bno; unsigned int size; - bool need_flush = true, split = false; ASSERT(atomic_read(&iclog->ic_refcnt) == 0); @@ -1925,10 +1921,8 @@ xlog_sync( bno = BLOCK_LSN(be64_to_cpu(iclog->ic_header.h_lsn)); /* Do we need to split this write into 2 parts? */ - if (bno + BTOBB(count) > log->l_logBBsize) { + if (bno + BTOBB(count) > log->l_logBBsize) xlog_split_iclog(log, &iclog->ic_header, bno, count); - split = true; - } /* calculcate the checksum */ iclog->ic_header.h_crc = xlog_cksum(log, &iclog->ic_header, @@ -1949,22 +1943,8 @@ xlog_sync( be64_to_cpu(iclog->ic_header.h_lsn)); } #endif - - /* - * Flush the data device before flushing the log to make sure all meta - * data written back from the AIL actually made it to disk before - * stamping the new log tail LSN into the log buffer. For an external - * log we need to issue the flush explicitly, and unfortunately - * synchronously here; for an internal log we can simply use the block - * layer state machine for preflushes. - */ - if (log->l_targ != log->l_mp->m_ddev_targp || split) { - blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev); - need_flush = false; - } - xlog_verify_iclog(log, iclog, count); - xlog_write_iclog(log, iclog, bno, count, need_flush); + xlog_write_iclog(log, iclog, bno, count); } /* @@ -2418,7 +2398,7 @@ xlog_write( ASSERT(log_offset <= iclog->ic_size - 1); ptr = iclog->ic_datap + log_offset; - /* start_lsn is the first lsn written to. That's all we need. */ + /* Start_lsn is the first lsn written to. */ if (start_lsn && !*start_lsn) *start_lsn = be64_to_cpu(iclog->ic_header.h_lsn); diff --git a/fs/xfs/xfs_log.h b/fs/xfs/xfs_log.h index 044e02cb8921..99f9d6ed9598 100644 --- a/fs/xfs/xfs_log.h +++ b/fs/xfs/xfs_log.h @@ -117,7 +117,6 @@ void xfs_log_mount_cancel(struct xfs_mount *); xfs_lsn_t xlog_assign_tail_lsn(struct xfs_mount *mp); xfs_lsn_t xlog_assign_tail_lsn_locked(struct xfs_mount *mp); void xfs_log_space_wake(struct xfs_mount *mp); -void xfs_log_release_iclog(struct xlog_in_core *iclog); int xfs_log_reserve(struct xfs_mount *mp, int length, int count, diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c index 172bb3551d6b..9d2fa8464289 100644 --- a/fs/xfs/xfs_log_cil.c +++ b/fs/xfs/xfs_log_cil.c @@ -890,15 +890,25 @@ xlog_cil_push_work( /* * If the checkpoint spans multiple iclogs, wait for all previous - * iclogs to complete before we submit the commit_iclog. + * iclogs to complete before we submit the commit_iclog. In this case, + * the commit_iclog write needs to issue a pre-flush so that the + * ordering is correctly preserved down to stable storage. */ + spin_lock(&log->l_icloglock); if (ctx->start_lsn != commit_lsn) { - spin_lock(&log->l_icloglock); xlog_wait_on_iclog(commit_iclog->ic_prev); + spin_lock(&log->l_icloglock); + commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH; } - /* release the hounds! */ - xfs_log_release_iclog(commit_iclog); + /* + * The commit iclog must be written to stable storage to guarantee + * journal IO vs metadata writeback IO is correctly ordered on stable + * storage. + */ + commit_iclog->ic_flags |= XLOG_ICL_NEED_FUA; + xlog_state_release_iclog(log, commit_iclog); + spin_unlock(&log->l_icloglock); return; out_skip: diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h index 56e1942c47df..2203ccecafb6 100644 --- a/fs/xfs/xfs_log_priv.h +++ b/fs/xfs/xfs_log_priv.h @@ -133,6 +133,9 @@ enum xlog_iclog_state { #define XLOG_COVER_OPS 5 +#define XLOG_ICL_NEED_FLUSH (1 << 0) /* iclog needs REQ_PREFLUSH */ +#define XLOG_ICL_NEED_FUA (1 << 1) /* iclog needs REQ_FUA */ + /* Ticket reservation region accounting */ #define XLOG_TIC_LEN_MAX 15 @@ -201,6 +204,7 @@ typedef struct xlog_in_core { u32 ic_size; u32 ic_offset; enum xlog_iclog_state ic_state; + unsigned int ic_flags; char *ic_datap; /* pointer to iclog data */ /* Callback structures need their own cacheline */ @@ -486,6 +490,8 @@ int xlog_commit_record(struct xlog *log, struct xlog_ticket *ticket, void xfs_log_ticket_ungrant(struct xlog *log, struct xlog_ticket *ticket); void xfs_log_ticket_regrant(struct xlog *log, struct xlog_ticket *ticket); +int xlog_state_release_iclog(struct xlog *log, struct xlog_in_core *iclog); + /* * When we crack an atomic LSN, we sample it first so that the value will not * change while we are cracking it into the component values. This means we