Horton

Re: XID formatting and SLRU refactorings

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Pavel Borisov <pashkin.elfe@gmail.com>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, Maxim Orlov <orlovmg@gmail.com>, Aleksander Alekseev <aleksander@timescale.com>, Postgres hackers <pgsql-hackers@lists.postgresql.org>, Japin Li <japinli@hotmail.com>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Date: 2023-11-28T10:37:08Z
Lists: pgsql-hackers 
On 28/11/2023 12:14, Pavel Borisov wrote:
> On Tue, 28 Nov 2023 at 13:13, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>
>> On 27/11/2023 01:43, Alexander Korotkov wrote:
>>> v61 looks good to me.  I'm going to push it as long as there are no objections.
>> This was discussed earlier, but is still present in v61:
>>
>>> +/*
>>> + * An internal function used by SlruScanDirectory().
>>> + *
>>> + * Returns true if a file with a name of a given length may be a correct
>>> + * SLRU segment.
>>> + */
>>> +static inline bool
>>> +SlruCorrectSegmentFilenameLength(SlruCtl ctl, size_t len)
>>> +{
>>> +     if (ctl->long_segment_names)
>>> +             return (len == 15); /* see SlruFileName() */
>>> +     else
>>> +             /*
>>> +              * Commit 638cf09e76d allowed 5-character lengths. Later commit
>>> +              * 73c986adde5 allowed 6-character length.
>>> +              *
>>> +              * XXX should we still consider such names to be valid?
>>> +              */
>>> +             return (len == 4 || len == 5 || len == 6);
>>> +}
>>> +
>>
>> I think it's pretty sloppy that the "short" filenames can be 4, 5 or 6
>> chars long. For pg_multixact/members, which introduced the 5-char case,
>> I think we should always pad the filenames 5 characters, and for
>> commit_ts which introduced the 6 char case, always pad to 6 characters.
>>
>> Instead of a "long_segment_names" boolean, how about an integer field,
>> to specify the length.
>>
>> That means that we'll need pg_upgrade to copy pg_multixact/members files
>> under the new names. That should be pretty straightforward.
> 
> I think what's done in patch 0001 is just an extension of existing
> logic and moving it into separate function.

That's right. I'm arguing that now is a good time to clean it up.

I won't insist if Alexander prefers to commit it as it is, though. But 
let's at least explain how this works in the comment, instead of the XXX.

-- 
Heikki Linnakangas
Neon (https://neon.tech)

Commits

  1. Fix more holes with SLRU code in need of int64 for segment numbers

  2. Use more consistently int64 for page numbers in SLRU-related code

  3. Improve comments in slru.{c,h} about segment name format

  4. Reorder actions in ProcArrayApplyRecoveryInfo()

  5. Fix typo in 5a1dfde8334b

  6. Fix warning due non-standard inline declaration in 4ed8f0913bfdb5f355

  7. Add SLRU tests for 64-bit page case

  8. Make use FullTransactionId in 2PC filenames

  9. Use larger segment file names for pg_notify

  10. Index SLRUs by 64-bit integers rather than by 32-bit integers

  11. Keep track of transaction commit timestamps

  12. Handle 5-char filenames in SlruScanDirectory