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Re: [PERFORM] Re: [PERFORM] Re: [PERFORM] Re: 回复: [PERFORM] PG as in-memory db? How to warm up and re-populate buffers? How to read in all tuples into memory?



On Tue, Feb 28, 2012 at 11:15 AM, Tomas Vondra <tv@xxxxxxxx> wrote:
> I haven't investigated why exactly the data are not cached initially, but
> none of the options that I can think of could be "fixed" by setting
> "fsync=off". That's something that influences writes (not read-only
> database) and I don't think it influences how buffers are evicted from
> shared buffers / page cache.
>
> It might speed up the initial load of data, but that's not what the OP was
> asking.

It speeds a lot more than the initial load of data.

Assuming the database is read-only, but not the filesystem (ie: it's
not a slave, in which case all this is moot, as you said, there are no
writes on a slave). That is, assuming this is a read-only master, then
read-only queries don't mean read-only filesystem. Bookkeeping tasks
like updating catalog dbs, statistics tables, page cleanup, stuff like
that can actually result in writes.

Writes that go through the WAL and then the filesystem.

With fsync=off, those writes happen on the background, and are carried
out by the OS. Effectively releasing postgres from having to wait on
them, and, assuming there's enough RAM, merging repeated writes to the
same sectors in one operation in the end. For stats, bookkeeping, and
who knows what else, the merging would be quite effective. With enough
RAM to hold the entire DB, the merging would effectively keep
everything in RAM (in system buffers) until there's enough I/O
bandwidth to transparently push that to persistent storage.

In essence, what was required, to keep everything in RAM for as much
as possible.

It *does* in the same way affect buffer eviction - it makes eviction
*very* quick, and re-population equally as quick, if everything fits
into memory.

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