Re: Compression of full-page-writes

On Fri, Oct 11, 2013 at 8:35 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> Hi,
> On 2013-10-11 03:44:01 +0900, Fujii Masao wrote:
>> I'm afraid that the patch has only limited effects in WAL reduction and
>> performance improvement unless the database contains highly-compressible
>> data like large blank characters column. It really depends on the contents
>> of the database. So, obviously FPW compression should not be the default.
>> Maybe we can treat it as just tuning knob.
>
>
> Have you tried using lz4 (or snappy) instead of pglz? There's a patch
> adding it to pg in
> http://archives.postgresql.org/message-id/20130621000900.GA12425%40alap2.anarazel.de

Yeah, it's worth checking them! Will do that.

> If this really is only a benefit in scenarios with lots of such data, I
> have to say I have my doubts about the benefits of the patch.

Yep, maybe the patch needs to be redesigned. Currently in the patch
compression is performed per FPW, i.e., the size of data to compress
is just 8KB. If we can increase the size of data to compress, we might
be able to improve the compression ratio. For example, by storing
all outstanding WAL data temporarily in local buffer, compressing them,
and then storing the compressed WAL data to WAL buffers.

Regards,

--
Fujii Masao