Sam Lightfoot created CASSANDRA-21679:
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Summary: Implement trickle_fsync with sync_file_range
Key: CASSANDRA-21679
URL: https://issues.apache.org/jira/browse/CASSANDRA-21679
Project: Apache Cassandra
Issue Type: Improvement
Components: Local/SSTable
Reporter: Sam Lightfoot
Assignee: Sam Lightfoot
h4. Summary
`trickle_fsync` paces SSTable writeback with `fdatasync`, which also commits
the ext4 journal, flushes the device cache on drives that have one, walks the
whole file's dirty set, and most notably blocks the flush writer with no
overlap between it and the device. None of that is required, durability is the
commitlog's job and `SequentialWriter` fsyncs at `close()` regardless.
The `sync_file_range` syscall does only the pacing work. This changes the
implementation beneath `trickle_fsync: true`; it does not add an option.
h4. What changes
At each interval, on Linux, two calls:
{code:java}
sync_file_range(fd, off_N, len, SYNC_FILE_RANGE_WRITE); // submit N, returns
sync_file_range(fd, off_N-1, len, SYNC_FILE_RANGE_WAIT_BEFORE); // wait for
N-1{code}
Lagging the wait by one interval is the point. `fdatasync` waits on data it
submitted in the same call, so producing and draining never overlap and every
interval costs both. Waiting one behind offsets the wait by exactly one
interval's generation time, N−1 has been in flight since the previous call, so
an interval costs the larger of the two and the stall is `max(0, drain −
produce)`.
That is zero whenever the device drains an interval faster than the writer
generates one, which is the case here at 24–33% device utilisation: 15.3 s →
1.0 s below, 93% of flush-writer stall removed, throughput and burst shaping
unchanged. Where the device is the limit instead, the writer is paced by it,
the backpressure `trickle_fsync` exists to provide. The writer runs at most one
interval ahead either way, so the bound holds.
`sync_file_range` writes no metadata, so the close sync becomes the writer's
only metadata commit.
h4. Measured
Single node, 10 MiB interval, ~411 MiB memtable flushes. Paired A/B, both arms
in the same invocation, n=5, arms matched to 0.0001% on read count.
| |trickle off|`fdatasync`|this change|
|flush-writer stall|0.4 s|15.3 s|1.0 s|
|ext4 journal transactions|3493|6574 (+3081)|3497 |
|read p99 during flush|24.3 ms|11.4 ms|9.0 ms|
|system flush throughput|116.4 MiB/s|116.4|116.5|
|dirty-window peak|439 MiB|50.8|48.6|
Stall drops 14.8× with no overlap between arms (14.2–17.0 s vs 0.8–1.4 s).
Flush-writer CPU is unchanged (69.7 vs 70.1 s), so that is time blocked, not
work removed.
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