[Added cross post to pgsql-hackers@lists.postgresql.org - background is
multi-TB database needs recovered via pgdumpall & reload, thoughts on
ways to make pg_dump scale to multi-thread to expedite loading to a new
cluster. Straight dump to a file is impractical as the dump will be
>200TB; hackers may be a better home for the discussion than current
admin list]
Hi Ron
On 18-Jul-2024 18:41, Ron Johnson wrote:
Multi-threaded writing to the same giant text file won't work too
well, when all the data for one table needs to be together.
Just temporarily add another disk for backups.
For clarity, I'm not proposing multi threaded writing to one file; the
proposal is a new special mode which specifically makes multiple output
streams across *network sockets* to a listener which is listening on the
other side. The goal is avoiding any files at all and only using
multiple network streams to gain multi-threaded processing with some
co-ordination to keep things organized and consistent.
This would really be specifically for the use-case of dump/reload
upgrade or recreate rather than everyday use. And particularly for very
large databases.
Looking at pg_dump.c it's doing the baseline organization but the
extension would be adding the required coordination with the
destination. So, for a huge table (I have many) these would go in
different streams but if there is a dependency (FK relations etc) the
checkpoint needs to ensure those are met before proceeding. Worst case
scenario it would end up using only 1 thread but it would be very
unusual to have a database where every table depends on another table
all the way down.
In theory at least, some gains should be achieved for typical databases
where a degree of parallelism is possible.
Thanks
Tom
On Thu, Jul 18, 2024 at 4:55 PM Thomas Simpson <t...@talentstack.to> wrote:
On 18-Jul-2024 16:32, Ron Johnson wrote:
On Thu, Jul 18, 2024 at 3:01 PM Thomas Simpson
<t...@talentstack.to> wrote:
[snip]
[BTW, v9.6 which I know is old but this server is stuck there]
[snip]
I know I'm stuck with the slow rebuild at this point.
However, I doubt I am the only person in the world that needs
to dump and reload a large database. My thought is this is a
weak point for PostgreSQL so it makes sense to consider ways
to improve the dump reload process, especially as it's the
last-resort upgrade path recommended in the upgrade guide and
the general fail-safe route to get out of trouble.
No database does fast single-threaded backups.
Agreed. My thought is that is should be possible for a 'new
dumpall' to be multi-threaded.
Something like :
* Set number of threads on 'source' (perhaps by querying a
listening destination for how many threads it is prepared to
accept via a control port)
* Select each database in turn
* Organize the tables which do not have references themselves
* Send each table separately in each thread (or queue them until a
thread is available) ('Stage 1')
* Rendezvous stage 1 completion (pause sending, wait until
feedback from destination confirming all completed) so we have a
known consistent state that is safe to proceed to subsequent tables
* Work through tables that do refer to the previously sent in the
same way (since the tables they reference exist and have their
data) ('Stage 2')
* Repeat progressively until all tables are done ('Stage 3', 4
etc. as necessary)
The current dumpall is essentially doing this table organization
currently [minus stage checkpoints/multi-thread] otherwise the
dump/load would not work. It may even be doing a lot of this for
'directory' mode? The change here is organizing n threads to
process them concurrently where possible and coordinating the
pipes so they only send data which can be accepted.
The destination would need to have a multi-thread listen and
co-ordinate with the sender on some control channel so feed back
completion of each stage.
Something like a destination host and control channel port to
establish the pipes and create additional netcat pipes on
incremental ports above the control port for each thread used.
Dumpall seems like it could be a reasonable start point since it
is already doing the complicated bits of serializing the dump data
so it can be consistently loaded.
Probably not really an admin question at this point, more a
feature enhancement.
Is there anything fundamentally wrong that someone with more
intimate knowledge of dumpall could point out?
Thanks
Tom
