Details
Minor - P4 Description
Journal writes are always compressed and then padded to 8kb. When the write is small enough (less than ~8kb today) then it doesn't need to be compressed. Compression adds latency and CPU overhead that makes some workloads slower.
By disabling compression I get ~10% more inserts/second for a workload that does journal writes which are less than 8k prior to compression – http://smalldatum.blogspot.com/2014/03/redo-logs-in-mongodb-and-innodb.html
This is likely to help workloads that use journalCommitInterval:2 and j:1 on writes.
A related JIRA is SERVER-9802.
void Journal::journal(const JSectHeader& h, const AlignedBuilder& uncompressed) {
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RACECHECK
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static AlignedBuilder b(32*1024*1024);
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/* buffer to journal will be
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JSectHeader
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compressed operations
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JSectFooter
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*/
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const unsigned headTailSize = sizeof(JSectHeader) + sizeof(JSectFooter);
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const unsigned max = maxCompressedLength(uncompressed.len()) + headTailSize;
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b.reset(max);
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{
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dassert( h.sectionLen() == (unsigned) 0xffffffff ); // we will backfill later
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b.appendStruct(h);
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}
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size_t compressedLength = 0;
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rawCompress(uncompressed.buf(), uncompressed.len(), b.cur(), &compressedLength);
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verify( compressedLength < 0xffffffff );
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verify( compressedLength < max );
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b.skip(compressedLength);
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Attachments
Issue Links
- related to
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SERVER-9802 Single-threaded journal compression becomes a bottleneck when using "durable" writes
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- Closed
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