Re: Compression of full-page-writes

> On Thu, Dec 4, 2014 at 8:37 PM, Michael Paquier wrote
> I pondered something that Andres mentioned upthread: we may not do the
>compression in WAL record only for blocks, but also at record level. Hence
>joining the two ideas together I think that we should definitely have
>a different
>GUC to control the feature, consistently for all the images. Let's call it
>wal_compression, with the following possible values:
>- on, meaning that a maximum of compression is done, for this feature
>basically full_page_writes = on.
>- full_page_writes, meaning that full page writes are compressed
>- off, default value, to disable completely the feature.
>This would let room for another mode: 'record', to completely compress
>a record. For now though, I think that a simple on/off switch would be
>fine for this patch. Let's keep things simple.

+1 for a separate parameter for compression

Some changed thoughts to the above

* parameter should be SUSET - it doesn't *need* to be set only at
server start since all records are independent of each other

* ideally we'd like to be able to differentiate the types of usage.
which then allows the user to control the level of compression
depending upon the type of action. My first cut at what those settings
should be are ALL > LOGICAL > PHYSICAL > VACUUM.

VACUUM - only compress while running vacuum commands
PHYSICAL - only compress while running physical DDL commands (ALTER
TABLE set tablespace, CREATE INDEX), i.e. those that wouldn't
typically be used for logical decoding
LOGICAL - compress FPIs for record types that change tables
ALL - all user commands
(each level includes all prior levels)

* name should not be wal_compression - we're not compressing all wal
records, just fpis. There is no evidence that we even want to compress
other record types, nor that our compression mechanism is effective at
doing so. Simple => keep name as compress_full_page_writes
Though perhaps we should have it called wal_compression_level

--
Simon Riggs http://www.2ndQuadrant.com/
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