Re: Different compression methods for FPI

On Tue, Jun 15, 2021 at 11:39:24AM +0900, Michael Paquier wrote:
> On Mon, Jun 14, 2021 at 08:42:08PM -0500, Justin Pryzby wrote:
>>>> It's USERSET following your own suggestion (which is a good suggestion):
> >> +       {"wal_compression_method", PGC_SIGHUP, WAL_SETTINGS,
> >> +           gettext_noop("Set the method used to compress full page images in the WAL."),
> >> +           NULL
> >> +       },
> >> +       &wal_compression_method,
> >> +       WAL_COMPRESSION_PGLZ, wal_compression_options,
> >> +       NULL, NULL, NULL
> >> Any reason to not make that user-settable?  If you merge that with
> >> wal_compression, that's not an issue.
> 
> Hmm, yeah.  This can be read as using PGC_USERSET.  With the second
> part of my sentence, I think that I imply to use PGC_SUSET and be
> consistent with wal_compression, but I don't recall my mood from one
> month ago :)  Sorry for the confusion.

Hold on - we're all confused (and I'm to blame).  The patch is changing the
existing wal_compression GUC, rather than adding wal_compression_method.
It's still SUSET, but in earlier messages, I called it USERSET, twice.

> You cannot do cross-checks for GUCs in their assign hooks or even rely
> in the order of those parameters, but you can do that in some backend
> code paths.  A recent discussion on the matter is for example what led
> to 79dfa8a for the GUCs controlling the min/max SSL protocols
> allowed.

Thank you - this is what I was remembering.

> > The goal of the patch is to give options, and the overhead of adding both zlib
> > and lz4 is low.  zlib gives good compression at some CPUcost and may be
> > preferable for (some) DWs, and lz4 is almost certainly better (than pglz) for
> > OLTPs.
> 
> Anything will be better than pglz.  I am rather confident in that.
> 
> What I am wondering is if we need to eat more bits than necessary for
> the WAL record format, because we will end up supporting it until the
> end of times.

Why ?  This is WAL, not table data.  WAL depends on the major version, so
I think wal_compression could provide a different set of compression methods at
every major release?

Actually, I was just thinking that default yes/no/on/off stuff maybe should be
defined to mean "lz4" rather than meaning pglz for "backwards compatibility".

> hear here, there are many cases that we may care about depending on
> how much CPU one is ready to pay in order to get more compression,
> knowing that there are no magic solutions for something that's cheap
> in CPU with a very good compression ratio or we could just go with
> that.  So it seems to me that there is still an argument for adding
> only one new compression method with a good range of levels, able to
> support the range of cases we'd care about:
> - High compression ratio but high CPU cost.
> - Low compression ratio but low CPU cost.

I think zlib is too expensive and lz4 doesn't get enough compression,
so neither supports both cases.  In a sample of 1, zlib-1 is ~35% slower than
lz4 and writes half as much.

I think zstd could support both cases; however, I still see it as this patch's
job to provide options, rather to definitively conclude which compression
algorithm is going to work best for everyone's use data and application.

-- 
Justin