Horton

Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-03-04T04:58:32Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Tighten the concurrent abort check during decoding.

  2. Improve hash_create()'s API for some added robustness.

  3. Use HASH_BLOBS for xidhash.

  4. Fix initialization of RelationSyncEntry for streaming transactions.

  5. Remove unused function declaration in logicalproto.h.

  6. Add additional tests to test streaming of in-progress transactions.

  7. Fix inline marking introduced in commit 464824323e.

  8. Add support for streaming to built-in logical replication.

  9. Fix the SharedFileSetUnregister API.

  10. Fix comment in procarray.c

  11. Suppress compiler warning in non-cassert builds.

  12. Extend the BufFile interface.

  13. Mark a few logical decoding related variables with PGDLLIMPORT.

  14. Implement streaming mode in ReorderBuffer.

  15. Extend the logical decoding output plugin API with stream methods.

  16. WAL Log invalidations at command end with wal_level=logical.

  17. Immediately WAL-log subtransaction and top-level XID association.

  18. Allow logical replication to transfer data in binary format.

  19. Only superuser can set sslcert/sslkey in postgres_fdw user mappings

  20. Track statistics for spilling of changes from ReorderBuffer.

  21. Add logical_decoding_work_mem to limit ReorderBuffer memory usage.

  22. logical decoding: process ASSIGNMENT during snapshot build

  23. Emit invalidations to standby for transactions without xid.

On Wed, Mar 4, 2020 at 3:16 AM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
>
> Hi,
>
> I started looking at this patch series again, hoping to get it moving
> for PG13.
>

It is good to keep moving this forward, but there are quite a few
problems with the design which need a broader discussion.  Some of
what I recall are:
a. Handling of abort of concurrent transactions.  There is some code
in the patch which might work, but there is not much discussion when
it was posted.
b. Handling of partial tuples (while streaming, we came to know that
toast tuple is not complete or speculative insert is incomplete).  For
this also, we have proposed a few solutions which need further
discussion.  One of those is implemented in the patch series.
c. We might also need some handling for replication origins.
d. Try to minimize the performance overhead of WAL logging for
invalidations.  We discussed different solutions for this and
implemented one of those.
e. How to skip already streamed transactions.

There might be a few more which I can't recall now.  Apart from this,
I haven't done any detailed review of subscriber-side implementation
where we write streamed transactions to file.  All of this will need
much more discussion and review before we can say it is ready to
commit, so I thought it might be better to pick it up for PG14 and
focus on other things that have a better chance for PG13 especially
because all the problems were not solved/discussed before last CF.
However, it is a good idea to keep moving this and have a discussion
on some of these issues.

> There's been a tremendous amount of work done since I last
> worked on it, and a lot was discussed on this thread, so it'll take a
> while to get familiar with the new code ...
>
> The first thing I realized that WAL-logging of assignments in v12 does
> both the "old" logging (using dedicated message) and "new" with
> toplevel-XID embedded in the first message. Yes, the patch was wrong,
> because it eliminated all calls to ProcArrayApplyXidAssignment() and so
> it was trivial to crash the replica due to KnownAssignedXids overflow.
> But I don't think re-introducing XLOG_XACT_ASSIGNMENT message is the
> right fix.
>
> I actually proposed doing this (having both ways to log assignments) so
> that there's no regression risk with (wal_level < logical). But IIRC
> Andres objected to it, argumenting that we should not log the same piece
> of information in two very different ways at the same time (IIRC it was
> discussed on the FOSDEM dev meeting, so I don't have a link to share).
> And I do agree with him ...
>

So, aren't we worried about the overhead of the amount of WAL and
performance impact for the transactions?  We might want to check the
pgbench read-write test to see if that will add any significant
overhead.

> The question is, why couldn't the replica use the same assignment info
> we already write for logical decoding?
>

I haven't thought about it in detail, but we can think on those lines
if the performance overhead is in the acceptable range.

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com