On 31 October 2017 at 07:05, Chris Travers <chris.travers@adjust.com> wrote:
> Hi;
>
> After Andres's excellent talk at PGConf we tried benchmarking
> effective_io_concurrency on some of our servers and found that those which
> have a number of NVME storage volumes could not fill the I/O queue even at
> the maximum setting (1000).
And was the system still i/o bound? If the cpu was 100% busy then
perhaps Postgres just can't keep up with the I/O system. It would
depend on workload though, if you start many very large sequential
scans you may be able to push the i/o system harder.
Keep in mind effective_io_concurrency only really affects bitmap index
scans (and to a small degree index scans). It works by issuing
posix_fadvise() calls for upcoming buffers one by one. That gets
multiple spindles active but it's not really going to scale to many
thousands of prefetches (and effective_io_concurrency of 1000 actually
means 7485 prefetches). At some point those i/o are going to start
completing before Postgres even has a chance to start processing the
data.
--
greg
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