Details
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Improvement
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Status: Resolved
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Major
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Resolution: Fixed
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None
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Description
As mutations written to a tablet server its buffered and once this buffer exceeds a certain size the data is dumped to the walog and then inserted into an in memory sorted map. These walog buffers are per a client and the max size is determined by tserver.mutation.queue.max.
Accumulo 1.5 and 1.6 call hsync() in hadoop 2 which ensures data is flushed to disk. This introduces a fixed delay when flushing walog buffers. The smaller tserver.mutation.queue.max is, the more frequently the walog buffers are flushed. With many clients writing to a tserver, this is not much of a concern because all of their walog buffers are flushed using group commit. This results in high throughput because large batches of data being written before hsync is called. However if a few client writing to a tserver there will be a lot more calls to hsync. It would be nice the # of calls to hsync was a function of the amount of data written regardless of the number of concurrent clients. Currently as the number of concurrent clients goes down, the number of calls to hsync goes up.
In 1.6 and 1.5 this can be mitigated by increasing tserver.mutation.queue.max, however this is multiplied by the number of concurrent writers. So increasing it can improve performance of a single writer but increases the chances of many concurrent writers exhausting memory.
Attachments
Issue Links
- is related to
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ACCUMULO-1905 WALog is slow
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- Resolved
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