Re: pg_walinspect - a new extension to get raw WAL data and WAL stats

On Mon, Jan 31, 2022 at 04:40:09PM -0500, Greg Stark wrote:
> 4) This isn't really an issue with your patch at all but why on earth
> do we have a bitvector for WAL compression methods?! Like, what does
> it mean to have multiple compression methods set? That should just be
> a separate field with values for each type of compression surely?

I don't have an answer to your question, but the discussion was here.

In the versions of the patches I sent on Mar 15, Mar 21, May 18, May 24, Jun
13, I avoided "one bit per compression method", but Michael thought this was
simpler.

https://www.postgresql.org/message-id/20210622031358.GF29179@telsasoft.com
On Mon, Jun 21, 2021 at 10:13:58PM -0500, Justin Pryzby wrote:
> +/* compression methods supported */
> +#define BKPIMAGE_COMPRESS_PGLZ 0x04
> +#define BKPIMAGE_COMPRESS_ZLIB 0x08
> +#define BKPIMAGE_COMPRESS_LZ4 0x10
> +#define BKPIMAGE_COMPRESS_ZSTD 0x20
> +#define BKPIMAGE_IS_COMPRESSED(info) \
> + ((info & (BKPIMAGE_COMPRESS_PGLZ | BKPIMAGE_COMPRESS_ZLIB | \
> + BKPIMAGE_COMPRESS_LZ4 | BKPIMAGE_COMPRESS_ZSTD)) != 0)
>
> You encouraged saving bits here, so I'm surprised to see that your patches
> use one bit per compression method: 2 bits to support no/pglz/lz4, 3 to add
> zstd, and the previous patch used 4 bits to also support zlib.
>
> There are spare bits available for that, but now there can be an inconsistency
> if two bits are set. Also, 2 bits could support 4 methods (including "no").

On Tue, Jun 22, 2021 at 12:53:46PM +0900, Michael Paquier wrote:
> Yeah, I know. I have just finished with that to get something
> readable for the sake of the tests. As you say, the point is moot
> just we add one new method, anyway, as we need just one new bit.
> And that's what I would like to do for v15 with LZ4 as the resulting
> patch is simple. It would be an idea to discuss more compression
> methods here once we hear more from users when this is released in the
> field, re-considering at this point if more is necessary or not.