Re: pg_restore mostly idle on restoring a large number of tables

On 7/13/23 02:41, Boris Sagadin wrote:
> Hi,
>
> restoring a 1.5TB database with about 800k tables on i3.4xlarge AWS
> instace, PgSQL V12.15 on Ubuntu.
>
> Running pg_restore with -j 16, I noticed the pg_restore is busy for an
> hour or so with IO at 80%+ and then most of processes start idling and
> only a few doing some work, disk IO at 1-2%, pg_stat_activity is mostly
> idle, same goes for CPU, and this state proceeds for further 6 hours, disk
> space increases very slowly.
>
> I thought because of a lot of small tables, number of workers should be
> increased to increase parallel efficiency, so I tried with -j 128. The
> situation was somewhat better, but most of the workers start idling, again
> disk IO lowers to about 4% util, CPU util goes to about 4%, too.
>
> Stracing workers produces the perpetual read call on most pg_restore workers:
>
> # strace -p 59567
> strace: Process 59567 attached
> read(3,
>
> With only about 10 or so (out of 128) workers doing some actual work:
>
> strace -p 59367 -e sendto
> strace: Process 59367 attached
> sendto(4, "Q\0\0\0\263ALTER TABLE ONLY raw.space0"..., 180, MSG_NOSIGNAL,
> NULL, 0) = 180
> sendto(4, "Q\0\0\0.TRUNCATE TABLE ONLY raw.spa"..., 47, MSG_NOSIGNAL,
> NULL, 0) = 47
> sendto(4, "Q\0\0\0\205COPY raw.space000448117 (da"..., 134, MSG_NOSIGNAL,
> NULL, 0) = sendto(4, "Q\0\0\0\vCOMMIT\0", 12, MSG_NOSIGNAL, NULL, 0) = 12
> sendto(4, "Q\0\0\0pCREATE INDEX idx_space00044"..., 113, MSG_NOSIGNAL,
> NULL, 0) = 113
> sendto(4, "Q\0\0\0\263ALTER TABLE ONLY raw.space0"..., 180, MSG_NOSIGNAL,
> NULL, 0) = 180
> ...
> .. some lines omitted
>
> I would think that all workers would proceed with creating indexes and
> doing some useful work until the restore is finished completely?
>
> Most of the tables are very small, 2 indexes per table and without any
> foreign references etc., we have a multi tenant environment.

I always run pg_dump and pg_restore with "--verbose", and redirect stdout &
stderr to a log file.  Then I "tail -f" that log file. It'll tell you what's
happening.

Also, iotop is quite useful.

--
Born in Arizona, moved to Babylonia.