On 23 Feb 2021 at 13:35, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> 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 this commit:
>
> commit 95d97c36e5155075ba2eb22b17562cfcc53fcf96
> Author: Steve Lord <lord@xxxxxxx>
> Date: Fri May 24 14:30:21 2002 +0000
>
> Add support for drive write cache flushing - should the kernel
> have the infrastructure
>
> 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.
>
> Given the new ordering semantics of commit records for the CIL, we
> need iclogs containing commit 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.
>
> 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
> 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@xxxxxxxxxx>
> ---
> Version 2:
> - repost manually without git/guilt mangling the patch author
> - fix bug in XLOG_ICL_NEED_FUA definition that didn't manifest as an
> ordering bug in generic/45[57] until testing the CIL pipelining
> changes much later in the series.
>
> fs/xfs/xfs_log.c | 33 +++++++++++++++++++++++----------
> fs/xfs/xfs_log_cil.c | 7 ++++++-
> fs/xfs/xfs_log_priv.h | 4 ++++
> 3 files changed, 33 insertions(+), 11 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index 6c3fb6dcb505..08d68a6161ae 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -1806,8 +1806,7 @@ 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);
>
> @@ -1845,10 +1844,12 @@ 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);
> @@ -1951,7 +1952,7 @@ xlog_sync(
> unsigned int roundoff; /* roundoff to BB or stripe */
> uint64_t bno;
> unsigned int size;
> - bool need_flush = true, split = false;
> + bool split = false;
>
> ASSERT(atomic_read(&iclog->ic_refcnt) == 0);
>
> @@ -2009,13 +2010,14 @@ xlog_sync(
> * 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) {
> + if (log->l_targ != log->l_mp->m_ddev_targp ||
> + (split && (iclog->ic_flags & XLOG_ICL_NEED_FLUSH))) {
> xfs_flush_bdev(log->l_mp->m_ddev_targp->bt_bdev);
> - need_flush = false;
> + iclog->ic_flags &= ~XLOG_ICL_NEED_FLUSH;
> }
If a checkpoint transaction spans across 2 or more iclogs and the log is
stored on an external device, then the above would remove XLOG_ICL_NEED_FLUSH
flag from iclog->ic_flags causing xlog_write_iclog() to include only REQ_FUA
flag in the corresponding bio.
Documentation/block/writeback_cache_control.rst seems to suggest that REQ_FUA
guarantees only that the data associated with the bio is stable on disk before
I/O completion is signalled. So looks like REQ_PREFLUSH is required in this
scenario.
--
chandan
