On 05 Mar 2021 at 10:41, 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 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 > 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. > I see that the missing preflush w.r.t previous iclogs of a multi-iclog checkpoint transaction has been handled in this version. Hence, Reviewed-by: Chandan Babu R <chandanrlinux@xxxxxxxxx> -- chandan