On 1/17/14, 7:57 AM, Robert Haas wrote:
- WAL files are written (and sometimes read) sequentially and fsync'd
very frequently and it's always good to write the data out to disk as
soon as possible
- Temp files are written and read sequentially and never fsync'd.
They should only be written to disk when memory pressure demands it
(but are a good candidate when that situation comes up)
- Data files are read and written randomly.  They are fsync'd at
checkpoint time; between checkpoints, it's best not to write them
sooner than necessary, but when the checkpoint arrives, they all need
to get out to the disk without bringing the system to a standstill

For sake of completeness... there are also data files that are temporary and 
don't need to be written to disk unless the kernel thinks there's better things 
to use that memory for. AFAIK those files are never fsync'd.

In other words, these are the same as the temp files Robert describes except 
they also have random access. Dunno if that matters.
Jim C. Nasby, Data Architect                       j...@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net

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