CommitLog writer should call fdatasync instead of fsync
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Key: CASSANDRA-3248
URL: https://issues.apache.org/jira/browse/CASSANDRA-3248
Project: Cassandra
Issue Type: Improvement
Components: Core
Affects Versions: 0.8.6, 1.0.0, 1.1
Environment: Linux
Reporter: Zhu Han
CommitLogSegment use SequentialWriter to flush the buffered data to log device.
It depends on FileDescriptor#sync() which invokes fsync() as it force the file
attributes to disk.
However, at least on Linux, fdatasync() is good enough for commit log flush:
bq. fdatasync() is similar to fsync(), but does not flush modified metadata
unless that metadata is needed in order to allow a subsequent data retrieval to
be correctly handled. For example, changes to st_atime or st_mtime
(respectively, time of last access and time of last modification; see stat(2))
do not require flushing because they are not necessary for a subsequent data
read to be handled correctly. On the other hand, a change to the file size
(st_size, as made by say ftruncate(2)), would require a metadata flush.
File size is synced to disk by fdatasync() either. Although the commit log
recovery logic sorts the commit log segements on their modify timestamp, it can
be removed safely, IMHO.
I checked the native code of JRE 6. On Linux and Solaris,
FileChannel#force(false) invokes fdatasync(). On windows, the false flag does
not have any impact.
On my log device (commodity SATA HDD, write cache disabled), fsync() and
fdatasync() has large performance gap:
$sysbench --test=fileio --num-threads=1 --file-num=1 --file-total-size=10G
--file-fsync-all=on --file-fsync-mode=fdatasync --file-test-mode=seqwr
--max-time=600 --file-block-size=2K --max-requests=0 run
54.90 Requests/sec executed
per-request statistics:
min: 8.29ms
avg: 18.18ms
max: 108.36ms
approx. 95 percentile: 25.02ms
$ sysbench --test=fileio --num-threads=1 --file-num=1 --file-total-size=10G
--file-fsync-all=on --file-fsync-mode=fsync --file-test-mode=seqwr
--max-time=600 --file-block-size=2K --max-requests=0 run
28.08 Requests/sec executed
per-request statistics:
min: 33.28ms
avg: 35.61ms
max: 911.87ms
approx. 95 percentile: 41.69ms
I do think this is a very critical performance improvement.
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