Re: Compression of full-page-writes - Mailing list pgsql-hackers

On Fri, Aug 30, 2013 at 8:25 AM, Fujii Masao <masao.fujii@gmail.com> wrote:
> Hi,
>
> Attached patch adds new GUC parameter 'compress_backup_block'.
> When this parameter is enabled, the server just compresses FPW
> (full-page-writes) in WAL by using pglz_compress() before inserting it
> to the WAL buffers. Then, the compressed FPW is decompressed
> in recovery. This is very simple patch.
>
> The purpose of this patch is the reduction of WAL size.
> Under heavy write load, the server needs to write a large amount of
> WAL and this is likely to be a bottleneck. What's the worse is,
> in replication, a large amount of WAL would have harmful effect on
> not only WAL writing in the master, but also WAL streaming and
> WAL writing in the standby. Also we would need to spend more
> money on the storage to store such a large data.
> I'd like to alleviate such harmful situations by reducing WAL size.
>
> My idea is very simple, just compress FPW because FPW is
> a big part of WAL. I used pglz_compress() as a compression method,
> but you might think that other method is better. We can add
> something like FPW-compression-hook for that later. The patch
> adds new GUC parameter, but I'm thinking to merge it to full_page_writes
> parameter to avoid increasing the number of GUC. That is,
> I'm thinking to change full_page_writes so that it can accept new value
> 'compress'.
>
> I measured how much WAL this patch can reduce, by using pgbench.
>
> * Server spec
>   CPU: 8core, Intel(R) Core(TM) i7-3630QM CPU @ 2.40GHz
>   Mem: 16GB
>   Disk: 500GB SSD Samsung 840
>
> * Benchmark
>   pgbench -c 32 -j 4 -T 900 -M prepared
>   scaling factor: 100
>
>   checkpoint_segments = 1024
>   checkpoint_timeout = 5min
>   (every checkpoint during benchmark were triggered by checkpoint_timeout)
>
> * Result
>   [tps]
>   1386.8 (compress_backup_block = off)
>   1627.7 (compress_backup_block = on)
>
>   [the amount of WAL generated during running pgbench]
>   4302 MB (compress_backup_block = off)
>   1521 MB (compress_backup_block = on)

This is really nice data.

I think if you want, you can once try with one of the tests Heikki has
posted for one of my other patch which is here:
http://www.postgresql.org/message-id/51366323.8070606@vmware.com

Also if possible, for with lesser clients (1,2,4) and may be with more
frequency of checkpoint.

This is just to show benefits of this idea with other kind of workload.

I think we can do these tests later as well, I had mentioned because
sometime back (probably 6 months), one of my colleagues have tried
exactly the same idea of using compression method (LZ and few others)
for FPW, but it turned out that even though the WAL size is reduced
but performance went down which is not the case in the data you have
shown even though you have used SSD, might be he has done some mistake
as he was not as experienced, but I think still it's good to check on
various workloads.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com