Thread: pg_restore enhancements

Thanks in advance for any suggestions or the green light to post this to the PG-developer group :-)

On 11/22/23 05:25, Efrain J. Berdecia wrote:
> After working for a site where we are constantly doing logical pg_dump 
> to refresh environments I've come to miss features available in other 
> RDBMS' refresh/restore utilities.
> 
> Someone could point me in the right direction otherwise, but pg_restore 
> seems to be lacking the ability to resume a restore upon failure, is all 
> or nothing with this guy. There also doesn't seem to be a way to control 
> batch size when doing the COPY phase, therefore preventing the WAL 
> directory from filling up and crashing the system.

The above needs more information on Postgres version(community or fork), 
OS and version, the size of the data set, the storage type and size, the 
Postgres conf, etc. Restores are being done all the time and this is the 
first report, as far as I can remember,  about an issue with COPY and 
WAL in a restore.


pg_restore
https://www.postgresql.org/docs/current/app-pgrestore.html

Does have:

--section=sectionname

     Only restore the named section. The section name can be pre-data, 
data, or post-data. This option can be specified more than once to 
select multiple sections. The default is to restore all sections.

     The data section contains actual table data as well as large-object 
definitions. Post-data items consist of definitions of indexes, 
triggers, rules and constraints other than validated check constraints. 
Pre-data items consist of all other data definition items.


AND

-l
--list

     List the table of contents of the archive. The output of this 
operation can be used as input to the -L option. Note that if filtering 
switches such as -n or -t are used with -l, they will restrict the items 
listed.


-L list-file
--use-list=list-file

     Restore only those archive elements that are listed in list-file, 
and restore them in the order they appear in the file. Note that if 
filtering switches such as -n or -t are used with -L, they will further 
restrict the items restored.

     list-file is normally created by editing the output of a previous 
-l operation. Lines can be moved or removed, and can also be commented 
out by placing a semicolon (;) at the start of the line. See below for 
examples.


>
> IMHO, it would be nice to have a feature that would allow pg_restore to 
> resume based on which part of the restore and/or object have already 
> been restored.
> 
> When it comes to the COPY phase of the restore, it would be nice to be 
> able to control batch size and resume COPY of a particular object upon 
> failure.

COPY as it stands now is all or none, so that command would have to be 
changed.

> 
> Thanks in advance for any suggestions or the green light to post this to 
> the PG-developer group :-)
> 
> Thanks,
> Efrain J. Berdecia

-- 
Adrian Klaver
adrian.klaver@aklaver.com

"Efrain J. Berdecia" <ejberdecia@yahoo.com> writes:
> Thanks, the issue we've run into, which I guess could be really a setup issue, with running a COPY command while
executingpg_restore, is that if we are restoring a large table (bigger than 500GB) our WAL directory can grow to be
verylarge. 
> I would think that if the pg_restore or COPY command was able to support a batch-size option, this should allow
postgresto either archive or remove wal files and prevent having to re-size the WAL directory for a one time refresh
operation.
> I'm trying to gage how feasible would be to start looking at contributing to add such a feature to either the COPY
commandor pg_restore. 

Given the shortage of other complaints, I tend to agree with Adrian
that there's not likely to be much interest in adding complexity
to pg_restore (or COPY) to address this.  You should probably look
harder at the idea that you have some configuration problem that's
triggering your WAL bloat.  If COPY can run you out of WAL space,
then so could any future bulk insert or update.

            regards, tom lane

On Wed, 2023-11-22 at 16:55 +0000, Efrain J. Berdecia wrote:
> Thanks, the issue we've run into, which I guess could be really a setup
> issue, with running a COPY command while executing pg_restore, 
> is that if we are restoring a large table (bigger than 500GB) our WAL directory can grow to be very large.

You can avoidwriting WAL if you set "wal_level = minimal", restart PostgreSQL
and restore the dump with the --single-transaction option.

Yours,
Laurenz Albe

Ron Johnson <ronljohnsonjr@gmail.com> writes:
> On Thu, Nov 23, 2023 at 3:37 AM Laurenz Albe <laurenz.albe@cybertec.at>
> wrote:
>> You can avoidwriting WAL if you set "wal_level = minimal", restart
>> PostgreSQL
>> and restore the dump with the --single-transaction option.

> Why does "--single-transaction" prevent WAL writes?  I'd expect _more_
> pg_wal growth from One Ginormous Transaction.

I don't recall all the details offhand, but there's some optimization
concerned with not writing WAL if COPY's target table was created in
the current transaction.  WAL will still be made for the catalog
changes, but usually the bulk of the WAL for a pg_restore run comes
from loading data, and this recipe eliminates that.  (Of course,
you cannot use it on a replication primary.)

            regards, tom lane