Re: XID formatting and SLRU refactorings (was: Add 64-bit XIDs into PostgreSQL 15)

Hi,

> Reviewing this now. I think it's almost ready to be committed.
>
> There's another big effort going on to move SLRUs to the regular buffer
> cache (https://commitfest.postgresql.org/43/3514/). I wonder how moving
> to 64 bit page numbers will affect that. BlockNumber is still 32 bits,
> after all.

Somehow I didn't pay too much attention to this effort, thanks. I will
familiarize myself with the patch. Intuitively I don't think that the
patchse should block each other.

> This patch makes the filenames always 12 characters wide (for SLRUs that
> opt-in to the new naming). That's actually not enough for the full range
> that a 64 bit page number can represent. Should we make it 16 characters
> now, to avoid repeating the same mistake we made earlier? Or make it
> more configurable, on a per-SLRU basis. One reason I don't want to just
> make it 16 characters is that WAL segment filenames are also 16 hex
> characters, which could cause confusion.

Good catch. I propose the following solution:

```
SlruFileName(SlruCtl ctl, char *path, int64 segno)
{
if (ctl->long_segment_names)
- return snprintf(path, MAXPGPATH, "%s/%012llX", ctl->Dir,
+ /*
+ * We could use 16 characters here but the disadvantage would be that
+ * the SLRU segments will be hard to distinguish from WAL segments.
+ *
+ * For this reason we use 15 characters. It is enough but also means
+ * that in the future we can't decrease SLRU_PAGES_PER_SEGMENT easily.
+ */
+ return snprintf(path, MAXPGPATH, "%s/%015llX", ctl->Dir,
(long long) segno);
else
return snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir,
```

PFE the corrected patchset v58.

--
Best regards,
Aleksander Alekseev