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: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Erik Rijkers <er@xs4all.nl>, Kuntal Ghosh <kuntalghosh.2007@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-06-02T11:25:58Z
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 Tue, Jun 2, 2020 at 3:59 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Thu, May 28, 2020 at 5:22 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> >
> > Also, what if the changes file size overflows "OS file size limit"?
> > If we agree that the above are problems then do you think we should
> > explore using BufFile interface (see storage/file/buffile.c) to avoid
> > all such problems?
>
> I also think that the file size is a problem.  I think we can use
> BufFile with some modifications.  We can not use the
> BufFileCreateTemp, because of few reasons
> 1) files get deleted on close, but we have to open/close on every
> stream start/stop.
> 2) even if we try to avoid closing we need to the BufFile pointers
> (which take 8192k per file) because there is no option to pass the
> file name.
>
> I thin for our use case BufFileCreateShared is more suitable.  I think
> we need to do some modifications so that we can use these apps without
> SharedFileSet. Otherwise, we need to unnecessarily need to create
> SharedFileSet for each transaction and also need to maintain it in xid
> array or xid hash until transaction commit/abort.  So I suggest
> following modifications in shared files set so that we can
> conveniently use it.
> 1. ChooseTablespace(const SharedFileSet fileset, const char name)
>   if fileset is NULL then select the DEFAULTTABLESPACEOID
> 2. SharedFileSetPath(char path, SharedFileSet fileset, Oid tablespace)
> If fileset is NULL then in directory path we can use MyProcPID or
> something instead of fileset->creator_pid.
>

Hmm, I find these modifications a bit ad-hoc.  So, not sure if it is
better than the patch maintains sharedfileset information.

> 3. Pass some parameter to BufFileOpenShared, so that it can open the
> file in RW mode instead of read-only mode.
>

This seems okay.

>
> > 2.
> > apply_handle_stream_abort()
> > {
> > ..
> > + /* discard the subxacts added later */
> > + nsubxacts = subidx;
> > +
> > + /* write the updated subxact list */
> > + subxact_info_write(MyLogicalRepWorker->subid, xid);
> > ..
> > }
> >
> > Here, if subxacts becomes zero, then also subxact_info_write will
> > create a new file and write checksum.
>
> How, will it create the new file, in fact it will write nsubxacts as 0
> in the existing file, and I think we need to do that right so that in
> next open we will know that the nsubxact is 0.
>
>   I think subxact_info_write
> > should have a check for nsubxacts > 0 before writing to the file.
>
> But, even if nsubxacts become 0 we want to write the file so that we
> can overwrite the previous info.
>

Can't we just remove the file for such a case?

apply_handle_stream_abort()
{
..
+ /* XXX optimize the search by bsearch on sorted data */
+ for (i = nsubxacts; i > 0; i--)
+ {
+ if (subxacts[i - 1].xid == subxid)
+ {
+ subidx = (i - 1);
+ found = true;
+ break;
+ }
+ }
+
+ /*
+ * If it's an empty sub-transaction then we will not find the subxid
+ * here so just free the memory and return.
+ */
+ if (!found)
+ {
+ /* Free the subxacts memory */
+ if (subxacts)
+ pfree(subxacts);
+
+ subxacts = NULL;
+ subxact_last = InvalidTransactionId;
+ nsubxacts = 0;
+ nsubxacts_max = 0;
+
+ return;
+ }
..
}

I have one question regarding the above code.  Isn't it possible that
a particular subtransaction id doesn't have any change but others do
we have?  For ex. cases like below:

postgres=# begin;
BEGIN
postgres=*# insert into t1 values(1);
INSERT 0 1
postgres=*# savepoint s1;
SAVEPOINT
postgres=*# savepoint s2;
SAVEPOINT
postgres=*# insert into t1 values(2);
INSERT 0 1
postgres=*# insert into t1 values(3);
INSERT 0 1
postgres=*# Rollback to savepoint s1;
ROLLBACK
postgres=*# commit;

Here, we have performed Rolledback to savepoint s1 which doesn't have
any change of its own.  I think this would have handled but just
wanted to confirm.

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