Horton

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

Tomas Vondra <tomas.vondra@2ndquadrant.com>

View thread
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-03-03T00:55:39Z
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.

Attachments

Hi there,

attached is an updated patch fixing all the reported issues (a bit more
about those below).

The main change in this patch version is reworked logging of subxact
assignments, which needs to be done immediately for incremental decoding
to work properly.

The previous patch versions did that by logging a separate xlog record,
which however had rather noticeable space overhead (~40% on a worst-case
test - tiny table, no FPWs, ...). While in practice the overhead would
be much closer to 0%, it still seemed unacceptable.

Andres proposed doing something like we do with replication origins in
XLogRecordAssemble, i.e. inventing a special block, and embedding the
assignment info into that (in the next xlog record). This turned out to
be working quite well, and the worst-case space overhead dropped to ~5%.

I have attempted to do something like that with the invalidations, which
is the other thing that needs to be logged immediately for incremental
decoding to work correctly. The plan was to use the same approach as for
assignments, i.e. embed the invalidations into the next xlog record and
stop sending them in the commit message. That however turned out to be
much more complicated - the embedding is fairly trivial, of course, but
unlike assignments the invalidations are needed for hot standbys. If we
only send them incrementally, I think the standby would have to collect
from the WAL records, and store them in a way that survives restarts.

So for invalidations the patch uses the original approach with a new
type xlog record type (ignored by standby), and still logging the
invalidations in commit record (which is that the standby relies on).


On 02/01/2018 11:50 PM, Tomas Vondra wrote:
> On 01/31/2018 07:53 AM, Masahiko Sawada wrote:
> ...
>> ----
>> CREATE SUBSCRIPTION commands accept work_mem < 64 but it leads ERROR
>> on publisher side when starting replication. Probably we should check
>> the value on the subscriber side as well.
>>

Added.

>> ----
>> When streaming = on, if we drop subscription in the middle of
>> receiving stream changes, DROP SUBSCRIPTION could leak tmp files
>> (.chages file and .subxacts file). Also it also happens when a
>> transaction on upstream aborted without abort record.
>>

Right. The files would get cleaned up eventually during restart (just
like other temporary files), but leaking them after DROP SUBSCRIPTION is
not cool. So I've added a simple tracking of files (or rather streamed
XIDs) in the worker, and clean them explicitly on exit.

>> ----
>> Since we can change both streaming option and work_mem option by ALTER
>> SUBSCRIPTION, documentation of ALTER SUBSCRIPTION needs to be updated.
>>

Yep, I've added note that work_mem and streaming can also be changed.
Those changes won't be applied to the already running worker, though.

>> ----
>> If we create a subscription without any options, both
>> pg_subscription.substream and pg_subscription.subworkmem are set to
>> null. However, since GetSubscription are't aware of NULL we start the
>> replication with invalid options like follows.
>> LOG:  received replication command: START_REPLICATION SLOT "hoge_sub"
>> LOGICAL 0/0 (proto_version '2', work_mem '893219954', streaming 'on',
>> publication_names '"hoge_pub"')
>>
>> I think we can set substream to false and subworkmem to -1 instead of
>> null, and then makes libpqrcv_startstreaming not set streaming option
>> if stream is -1.
>>

Good catch! I've done pretty much what you suggested here, i.e. store
-1/false instead and then handle that in libpqrcv_startstreaming.

>> ----
>> Some WARNING messages appeared. Maybe these are for debug purpose?
>>
>> WARNING:  updating stream stats 0x1c12ef8 4 3 65604
>> WARNING: UpdateSpillStats: updating stats 0x1c12ef8 0 0 0 39 41 2632080
>>

Yeah, those should be removed.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services