Hi,
There is another problem.
When the autovacuum recycles the old pages, the ReadBuffer_common()
would do mdread() instead of mdextend().
The read is synchronous, while the write could be mostly asynchronous,
so the frequent read is much worse than write version.
Any help? Please.
Regards,
Jinhua Luo
2016-01-19 12:50 GMT+08:00 Jinhua Luo <luajit.io@gmail.com>:
> Hi,
>
> I thought with async commit enabled, the backend process would rarely
> do file io. But in fact, it still involves a lot of file io.
>
> After inspecting the vfs probes using systemtap, and reading the
> source codes of postgresql, I found the tight loop of insert or update
> will cause heavy file io upon the data files (table, indexes) directly
> by the backend process! And those io has nothing to do with shared
> buffer dirty writes.
>
> The heap_insert() or heap_update() would invoke
> RelationGetBufferForTuple(), which in turn finally invoke
> ReadBuffer_common():
>
> 1) lookup or allocate the buffer from shared buffer, which may cause
> dirty write (but in my case, it's rare. Maybe the shared buffer is big
> enough and the checkpointer or bgwriter always clean it in time). If
> the target buffer is found, skip following steps.
>
> 2) if it needs to extend the relation (insert or update on new table
> would normally fall in this case), then it would write zero-filled
> page into the disk (used to occupy the file space? But most file
> systems support file hole or space reservation, so maybe this part
> could be optimized?) This procedure would hold the exclusive lock on
> the relation. So if the write is slow, it would slow down all pending
> queries of the lock waiters.
>
> 3) Otherwise, it would read from disk.
>
> The target buffer would be locked exclusively until the insert or
> update finish. Note that the insert or update also involve xlog
> insert, although with async commit enabled, the backend process would
> not flush the xlog, but chances are that the xlog buffer dirty writes
> happens (although it's also rare in my case).
>
> So I guess the real reason is the file io with lock holding. If io
> spike happens, it would cause long query duration.
>
> Am I correct? Look forward to any advice.
>
> Thanks.
>
> Regards,
> Jinhua Luo