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xfs: journal IO cache flush reductions
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 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> Reviewed-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Allison Henderson <allison.henderson@oracle.com> Signed-off-by: Darrick J. Wong <djwong@kernel.org>
This commit is contained in:
parent
3468bb1ca6
commit
eef983ffea
4 changed files with 43 additions and 48 deletions
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@ -513,7 +513,7 @@ __xlog_state_release_iclog(
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* Flush iclog to disk if this is the last reference to the given iclog and the
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* it is in the WANT_SYNC state.
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*/
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static int
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int
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xlog_state_release_iclog(
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struct xlog *log,
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struct xlog_in_core *iclog)
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@ -533,23 +533,6 @@ xlog_state_release_iclog(
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return 0;
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}
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void
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xfs_log_release_iclog(
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struct xlog_in_core *iclog)
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{
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struct xlog *log = iclog->ic_log;
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bool sync = false;
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if (atomic_dec_and_lock(&iclog->ic_refcnt, &log->l_icloglock)) {
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if (iclog->ic_state != XLOG_STATE_IOERROR)
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sync = __xlog_state_release_iclog(log, iclog);
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spin_unlock(&log->l_icloglock);
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}
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if (sync)
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xlog_sync(log, iclog);
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}
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/*
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* Mount a log filesystem
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*
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@ -837,6 +820,14 @@ xlog_write_unmount_record(
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/* account for space used by record data */
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ticket->t_curr_res -= sizeof(ulf);
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/*
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* For external log devices, we need to flush the data device cache
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* first to ensure all metadata writeback is on stable storage before we
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* stamp the tail LSN into the unmount record.
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*/
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if (log->l_targ != log->l_mp->m_ddev_targp)
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blkdev_issue_flush(log->l_targ->bt_bdev);
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return xlog_write(log, &vec, ticket, NULL, NULL, XLOG_UNMOUNT_TRANS);
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}
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@ -874,6 +865,11 @@ xlog_unmount_write(
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else
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ASSERT(iclog->ic_state == XLOG_STATE_WANT_SYNC ||
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iclog->ic_state == XLOG_STATE_IOERROR);
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/*
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* Ensure the journal is fully flushed and on stable storage once the
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* iclog containing the unmount record is written.
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*/
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iclog->ic_flags |= (XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA);
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error = xlog_state_release_iclog(log, iclog);
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xlog_wait_on_iclog(iclog);
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@ -1755,8 +1751,7 @@ xlog_write_iclog(
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struct xlog *log,
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struct xlog_in_core *iclog,
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uint64_t bno,
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unsigned int count,
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bool need_flush)
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unsigned int count)
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{
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ASSERT(bno < log->l_logBBsize);
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@ -1794,10 +1789,12 @@ xlog_write_iclog(
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* writeback throttle from throttling log writes behind background
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* metadata writeback and causing priority inversions.
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*/
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iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC |
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REQ_IDLE | REQ_FUA;
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if (need_flush)
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iclog->ic_bio.bi_opf = REQ_OP_WRITE | REQ_META | REQ_SYNC | REQ_IDLE;
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if (iclog->ic_flags & XLOG_ICL_NEED_FLUSH)
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iclog->ic_bio.bi_opf |= REQ_PREFLUSH;
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if (iclog->ic_flags & XLOG_ICL_NEED_FUA)
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iclog->ic_bio.bi_opf |= REQ_FUA;
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iclog->ic_flags &= ~(XLOG_ICL_NEED_FLUSH | XLOG_ICL_NEED_FUA);
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if (xlog_map_iclog_data(&iclog->ic_bio, iclog->ic_data, count)) {
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xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
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@ -1900,7 +1897,6 @@ xlog_sync(
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unsigned int roundoff; /* roundoff to BB or stripe */
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uint64_t bno;
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unsigned int size;
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bool need_flush = true, split = false;
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ASSERT(atomic_read(&iclog->ic_refcnt) == 0);
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@ -1925,10 +1921,8 @@ xlog_sync(
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bno = BLOCK_LSN(be64_to_cpu(iclog->ic_header.h_lsn));
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/* Do we need to split this write into 2 parts? */
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if (bno + BTOBB(count) > log->l_logBBsize) {
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if (bno + BTOBB(count) > log->l_logBBsize)
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xlog_split_iclog(log, &iclog->ic_header, bno, count);
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split = true;
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}
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/* calculcate the checksum */
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iclog->ic_header.h_crc = xlog_cksum(log, &iclog->ic_header,
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@ -1949,22 +1943,8 @@ xlog_sync(
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be64_to_cpu(iclog->ic_header.h_lsn));
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}
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#endif
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/*
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* Flush the data device before flushing the log to make sure all meta
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* data written back from the AIL actually made it to disk before
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* stamping the new log tail LSN into the log buffer. For an external
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* log we need to issue the flush explicitly, and unfortunately
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* synchronously here; for an internal log we can simply use the block
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* layer state machine for preflushes.
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*/
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if (log->l_targ != log->l_mp->m_ddev_targp || split) {
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blkdev_issue_flush(log->l_mp->m_ddev_targp->bt_bdev);
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need_flush = false;
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}
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xlog_verify_iclog(log, iclog, count);
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xlog_write_iclog(log, iclog, bno, count, need_flush);
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xlog_write_iclog(log, iclog, bno, count);
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}
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/*
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@ -2418,7 +2398,7 @@ xlog_write(
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ASSERT(log_offset <= iclog->ic_size - 1);
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ptr = iclog->ic_datap + log_offset;
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/* start_lsn is the first lsn written to. That's all we need. */
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/* Start_lsn is the first lsn written to. */
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if (start_lsn && !*start_lsn)
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*start_lsn = be64_to_cpu(iclog->ic_header.h_lsn);
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@ -117,7 +117,6 @@ void xfs_log_mount_cancel(struct xfs_mount *);
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xfs_lsn_t xlog_assign_tail_lsn(struct xfs_mount *mp);
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xfs_lsn_t xlog_assign_tail_lsn_locked(struct xfs_mount *mp);
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void xfs_log_space_wake(struct xfs_mount *mp);
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void xfs_log_release_iclog(struct xlog_in_core *iclog);
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int xfs_log_reserve(struct xfs_mount *mp,
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int length,
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int count,
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@ -890,15 +890,25 @@ xlog_cil_push_work(
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/*
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* If the checkpoint spans multiple iclogs, wait for all previous
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* iclogs to complete before we submit the commit_iclog.
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* iclogs to complete before we submit the commit_iclog. In this case,
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* the commit_iclog write needs to issue a pre-flush so that the
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* ordering is correctly preserved down to stable storage.
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*/
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spin_lock(&log->l_icloglock);
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if (ctx->start_lsn != commit_lsn) {
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spin_lock(&log->l_icloglock);
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xlog_wait_on_iclog(commit_iclog->ic_prev);
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spin_lock(&log->l_icloglock);
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commit_iclog->ic_flags |= XLOG_ICL_NEED_FLUSH;
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}
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/* release the hounds! */
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xfs_log_release_iclog(commit_iclog);
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/*
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* The commit iclog must be written to stable storage to guarantee
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* journal IO vs metadata writeback IO is correctly ordered on stable
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* storage.
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*/
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commit_iclog->ic_flags |= XLOG_ICL_NEED_FUA;
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xlog_state_release_iclog(log, commit_iclog);
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spin_unlock(&log->l_icloglock);
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return;
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out_skip:
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@ -133,6 +133,9 @@ enum xlog_iclog_state {
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#define XLOG_COVER_OPS 5
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#define XLOG_ICL_NEED_FLUSH (1 << 0) /* iclog needs REQ_PREFLUSH */
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#define XLOG_ICL_NEED_FUA (1 << 1) /* iclog needs REQ_FUA */
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/* Ticket reservation region accounting */
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#define XLOG_TIC_LEN_MAX 15
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@ -201,6 +204,7 @@ typedef struct xlog_in_core {
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u32 ic_size;
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u32 ic_offset;
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enum xlog_iclog_state ic_state;
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unsigned int ic_flags;
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char *ic_datap; /* pointer to iclog data */
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/* Callback structures need their own cacheline */
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@ -486,6 +490,8 @@ int xlog_commit_record(struct xlog *log, struct xlog_ticket *ticket,
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void xfs_log_ticket_ungrant(struct xlog *log, struct xlog_ticket *ticket);
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void xfs_log_ticket_regrant(struct xlog *log, struct xlog_ticket *ticket);
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int xlog_state_release_iclog(struct xlog *log, struct xlog_in_core *iclog);
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/*
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* When we crack an atomic LSN, we sample it first so that the value will not
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* change while we are cracking it into the component values. This means we
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