Re: Deleting bytea, autovacuum, and 8.2/8.4 differences

A quick test:
-----
1. create table x1(x int, y bytea);
2. Load some data say with python:
cp /opt/java/src.zip ~/tmp/a.dat (19MB)
##
import psycopg2
conn = psycopg2.connect("dbname='test' user='*****' password='****'
host='127.0.0.1'");
conn.cursor().execute("INSERT INTO x1 VALUES (1, %s)",
(psycopg2.Binary(open("a.dat").read()),))
conn.commit()
##
3. create table x2(x int, y bytea);
4. Copy table x1 100 times to x2 (1.9GB) and monitor/measure IO:
insert into x2 select a x, y from generate_series(1,100) a, x1;

Results:
-----------
On Linux 2.6.32 with an ext3 file system on one 15K rpm disk, we saw with
SystemTap that the source 1.9GB (19MB x 100) resulted in 5GB of actual disk
IO and took 61 seconds (52 CPU + 9 sleep/wait for IO)

Deletion (delete from x2) took 32 seconds with 12 seconds CPU and 20 sec
sleep + wait for IO. Actual disk IO was about 4GB.

Since Pg does not use the concept of rollback segments, it is unclear why
deletion produces so much disk IO (4GB).

VJ

On Sat, Mar 13, 2010 at 5:17 PM, fkater(at)googlemail(dot)com <
fkater(at)googlemail(dot)com> wrote:

> Hi all,
>
> my posting on 2010-01-14 about the performance when writing
> bytea to disk caused a longer discussion. While the fact
> still holds that the overall postgresql write performance is
> roughly 25% of the serial I/O disk performance this was
> compensated for my special use case here by doing some other
> non-postgresql related things in parallel.
>
> Now I cannot optimize my processes any further, however, now
> I am facing another quite unexpected performance issue:
> Deleting rows from my simple table (with the bytea column)
> having 16 MB data each, takes roughly as long as writing
> them!
>
> Little more detail:
>
> * The table just has 5 unused int columns, a timestamp,
> OIDs, and the bytea column, no indices; the bytea storage
> type is 'extended', the 16 MB are compressed to approx. the
> half.
>
> * All the usual optimizations are done to reach better
> write through (pg_xlog on another disk, much tweaks to the
> server conf etc), however, this does not matter here, since
> not the absolute performance is of interest here but the
> fact that deleting roughly takes 100% of the writing time.
>
> * I need to write 15 rows of 16 MB each to disk in a maximum
> time of 15 s, which is performed here in roughly 10 seconds,
> however, now I am facing the problem that keeping my
> database tidy (deleting rows) takes another 5-15 s (10s on
> average), so my process exceeds the maximum time of 15s for
> about 5s.
>
> * Right now I am deleting like this:
>
> DELETE FROM table WHERE (CURRENT_TIMESTAMP -
> my_timestamp_column) > interval '2 minutes';
>
> while it is planned to have the interval set to 6 hours in
> the final version (thus creating a FIFO buffer for the
> latest 6 hours of inserted data; so the FIFO will keep
> approx. 10.000 rows spanning 160-200 GB data).
>
> * This deletion SQL command was simply repeatedly executed
> by pgAdmin while my app kept adding the 16 MB rows.
>
> * Autovacuum is on; I believe I need to keep it on,
> otherwise I do not free the disk space, right? If I switch
> it off, the deletion time reduces from the average 10s down
> to 4s.
>
> * I am using server + libpq version 8.2.4, currently on
> WinXP. Will an upgrade to 8.4 help here?
>
> Do you have any other ideas to help me out?
> Oh, please...
>
> Thank You
> Felix
>
>
>
>
>
> --
> Sent via pgsql-performance mailing list (pgsql-performance(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-performance
>