Slow pg_dump

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We're having what seem like serious performance issues with pg_dump, and =
I hope someone can help. =20

We have several tables that are used to store binary data as bytea (in =
this example image files), but we're having similar time issues with =
text tables as well.

In my most recent test, the sample table was about 5 GB in 1644 rows, =
with image files sizes between 1 MB and 35 MB. The server was a 3.0 GHz =
P4 running WinXP, with 2 GB of ram, the backup stored to a separate disk =
from the data, and little else running on the sytem.

We're doing the following:
=20
pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f =
"backupTest.backup" -t "public"."images" db_name

In the test above, this took 1hr 45min to complete. Since we expect to =
have users with 50-100GB of data, if not more, backup times that take =
nearly an entire day are unacceptable.

We think there must be something we're doing wrong. A search turned up a =
similar thread =
(http://archives.postgresql.org/pgsql-performance/2007-12/ms g00404.php), =
but our number are so much higher than those that we must be doing =
something very wrong. Hopefully, either there's a server setting or =
pg_dump option we need to change, but we're open to design changes if =
necessary.

Can anyone who has dealt with this before advise us?

Thanks!
Ryan


=20




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charset=3Diso-8859-1">
6.5.7652.24">

We're having what seem like serious performance issues =
with pg_dump, and I hope someone can help. 



We have several tables that are used to store binary data as bytea (in =
this example image files), but we're having similar time issues with =
text tables as well.



In my most recent test, the sample table was about 5 GB in 1644 rows, =
with image files sizes between 1 MB and 35 MB.  The server was a =
3.0 GHz P4 running WinXP, with 2 GB of ram, the backup stored to a =
separate disk from the data, and little else running on the sytem.



We're doing the following:



pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f =
"backupTest.backup" -t "public"."images" =
db_name



In the test above, this took 1hr 45min to complete.  Since we =
expect to have users with 50-100GB of data, if not more, backup times =
that take nearly an entire day are unacceptable.



We think there must be something we're doing wrong. A search turned up a =
similar thread ( HREF=3D"http://archives.postgresql.org/pgsql-performance/200 7-12/msg00404=
..php">http://archives.postgresql.org/pgsql-performance/2007 -12/msg00404.p=
hp), but our number are so much higher than those that we must be =
doing something very wrong.  Hopefully, either there's a server =
setting or pg_dump option we need to change, but we're open to design =
changes if necessary.



Can anyone who has dealt with this before advise us?



Thanks!

Ryan

Re: Slow pg_dump

"Ryan Wells" writes:
> We have several tables that are used to store binary data as bytea (in
> this example image files),

Precompressed image formats, no doubt?

> pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f "backupTest.backup" -t "public"."images" db_name

Try it with -Z0, or even drop the -Fc completely, since it's certainly
not very helpful on a single-table dump. Re-compressing already
compressed data is not only useless but impressively slow ...

Also, drop the -i, that's nothing but a foot-gun.

regards, tom lane

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Re: Slow pg_dump

The images are stored in whatever format our users load them as, so we
don't have any control over their compression or lack thereof.

I ran pg_dump with the arguments you suggested, and my 4 GB test table
finished backing up in about 25 minutes, which seems great. The only
problem is that the resulting backup file was over 9 GB. Using -Z2
resulting in a 55 minute 6GB backup.

Here's my interpretation of those results: the TOAST tables for our
image files are compressed by Postgres. During the backup, pg_dump
uncompresses them, and if compression is turned on, recompresses the
backup. Please correct me if I'm wrong there.

If we can't find a workable balance using pg_dump, then it looks like
our next best alternative may be a utility to handle filesystem backups,
which is a little scary for on-site, user-controlled servers.

Ryan

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]=20
Sent: Saturday, April 12, 2008 9:46 PM
To: Ryan Wells
Cc: pgsql-admin@postgresql.org
Subject: Re: [ADMIN] Slow pg_dump=20

"Ryan Wells" writes:
> We have several tables that are used to store binary data as bytea (in

> this example image files),

Precompressed image formats, no doubt?
=20
> pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f=20
> "backupTest.backup" -t "public"."images" db_name

Try it with -Z0, or even drop the -Fc completely, since it's certainly
not very helpful on a single-table dump. Re-compressing already
compressed data is not only useless but impressively slow ...

Also, drop the -i, that's nothing but a foot-gun.

regards, tom lane

--=20
Sent via pgsql-admin mailing list (pgsql-admin@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-admin

Re: Slow pg_dump

> I ran pg_dump with the arguments you suggested, and my 4 GB test table
finished
> backing up in about 25 minutes, which seems great. The only problem is
that the
> resulting backup file was over 9 GB. Using -Z2 resulting in a 55 minute
6GB backup.
>=20
> Here's my interpretation of those results: the TOAST tables for our image
files
> are compressed by Postgres. During the backup, pg_dump uncompresses them,
and if
> compression is turned on, recompresses the backup. Please correct me if
I'm wrong
> there.
>=20
> If we can't find a workable balance using pg_dump, then it looks like our
next
> best alternative may be a utility to handle filesystem backups, which is a
littlescary for on-site, user-controlled servers.

Ryan

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]=20
Sent: Saturday, April 12, 2008 9:46 PM
To: Ryan Wells
Cc: pgsql-admin@postgresql.org
Subject: Re: [ADMIN] Slow pg_dump=20

"Ryan Wells" writes:
> We have several tables that are used to store binary data as bytea (in

> this example image files),

Precompressed image formats, no doubt?
=20
> pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f
> "backupTest.backup" -t "public"."images" db_name

Try it with -Z0, or even drop the -Fc completely, since it's certainly not
very helpful on a single-table dump. Re-compressing already compressed data
is not only useless but impressively slow ...

Also, drop the -i, that's nothing but a foot-gun.

regards, tom lane

--=20
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changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin


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Re: Slow pg_dump

(Sorry, hit send too soon!)

> I ran pg_dump with the arguments you suggested, and my 4 GB test
> table finished backing up in about 25 minutes, which seems great.
> The only problem is that the resulting backup file was over 9 GB.
> Using -Z2 resulting in a 55 minute 6GB backup.
>
> Here's my interpretation of those results: the TOAST tables for
> our image files are compressed by Postgres. During the backup,
> pg_dump uncompresses them, and if compression is turned on,
> recompresses the backup. Please correct me if I'm wrong there.
>
> If we can't find a workable balance using pg_dump, then it looks
> like our next best alternative may be a utility to handle
> filesystem backups, which is a little scary for on-site,
> user-controlled servers.

How about a post-backup compress?
pg_dump -Z0 > uncompressed-backup.sql
gzip uncompressed-backup.sql
mv uncompressed-backup.sql.gz compressed-backup.sql.gz

Your backup is completed in reasonable time, you're just handling
the storage of the backup afterwards, while users can be using the
System again...


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Re: Slow pg_dump

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Hi,

I have never dealt with tables that are made
of compressed data, but I back up the database
via crontab file like this:


.
.
filename=3D`date +%G%m%d.%w`.gz
/usr/local/pgsql/bin/pg_dumpall | gzip > /some_destination/$filename
.
.

Hope this helps.

Tena Sakai
tsakai@gallo.ucsf.edu


-----Original Message-----
From: pgsql-admin-owner@postgresql.org on behalf of Ryan Wells
Sent: Sat 4/12/2008 5:59 PM
To: Ryan Wells; pgsql-admin@postgresql.org
Subject: [ADMIN] Slow pg_dump
=20

We're having what seem like serious performance issues with pg_dump, and =
I hope someone can help. =20

We have several tables that are used to store binary data as bytea (in =
this example image files), but we're having similar time issues with =
text tables as well.

In my most recent test, the sample table was about 5 GB in 1644 rows, =
with image files sizes between 1 MB and 35 MB. The server was a 3.0 GHz =
P4 running WinXP, with 2 GB of ram, the backup stored to a separate disk =
from the data, and little else running on the sytem.

We're doing the following:
=20
pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f =
"backupTest.backup" -t "public"."images" db_name

In the test above, this took 1hr 45min to complete. Since we expect to =
have users with 50-100GB of data, if not more, backup times that take =
nearly an entire day are unacceptable.

We think there must be something we're doing wrong. A search turned up a =
similar thread =
(http://archives.postgresql.org/pgsql-performance/2007-12/ms g00404.php), =
but our number are so much higher than those that we must be doing =
something very wrong. Hopefully, either there's a server setting or =
pg_dump option we need to change, but we're open to design changes if =
necessary.

Can anyone who has dealt with this before advise us?

Thanks!
Ryan


=20





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charset=3Diso-8859-1">
6.5.7651.59">

Hi,



I have never dealt with tables that are made

of compressed data, but I back up the database

via crontab file like this:



     <some envrironment variable setup>

            &=
nbsp; .

            &=
nbsp; .

  filename=3D`date +%G%m%d.%w`.gz

  /usr/local/pgsql/bin/pg_dumpall | gzip > =
/some_destination/$filename

            &=
nbsp; .

            &=
nbsp; .



Hope this helps.



Tena Sakai

tsakai@gallo.ucsf.edu





-----Original Message-----

From: pgsql-admin-owner@postgresql.org on behalf of Ryan Wells

Sent: Sat 4/12/2008 5:59 PM

To: Ryan Wells; pgsql-admin@postgresql.org

Subject: [ADMIN] Slow pg_dump





We're having what seem like serious performance issues with pg_dump, and =
I hope someone can help. 



We have several tables that are used to store binary data as bytea (in =
this example image files), but we're having similar time issues with =
text tables as well.



In my most recent test, the sample table was about 5 GB in 1644 rows, =
with image files sizes between 1 MB and 35 MB.  The server was a =
3.0 GHz P4 running WinXP, with 2 GB of ram, the backup stored to a =
separate disk from the data, and little else running on the sytem.



We're doing the following:



pg_dump -i -h localhost -p 5432 -U postgres -F c -v -f =
"backupTest.backup" -t "public"."images" =
db_name



In the test above, this took 1hr 45min to complete.  Since we =
expect to have users with 50-100GB of data, if not more, backup times =
that take nearly an entire day are unacceptable.



We think there must be something we're doing wrong. A search turned up a =
similar thread ( HREF=3D"http://archives.postgresql.org/pgsql-performance/200 7-12/msg00404=
..php">http://archives.postgresql.org/pgsql-performance/2007 -12/msg00404.p=
hp), but our number are so much higher than those that we must be =
doing something very wrong.  Hopefully, either there's a server =
setting or pg_dump option we need to change, but we're open to design =
changes if necessary.



Can anyone who has dealt with this before advise us?



Thanks!

Ryan

Re: Slow pg_dump

"Phillip Smith" writes:
>> Here's my interpretation of those results: the TOAST tables for
>> our image files are compressed by Postgres. During the backup,
>> pg_dump uncompresses them, and if compression is turned on,
>> recompresses the backup. Please correct me if I'm wrong there.

No, the TOAST tables aren't compressed, they're pretty much going to be
the raw image data (plus a bit of overhead).

What I think is happening is that COPY OUT is encoding the bytea
data fairly inefficiently (one byte could go to \\nnn, five bytes)
and the compression on the pg_dump side isn't doing very well at buying
that back.

I experimented a bit and noticed that pg_dump -Fc is a great deal
smarter about storing large objects than big bytea fields --- it seems
to be pretty nearly one-to-one with the original data size when storing
a compressed file that was put into a large object. I dunno if it's
practical for you to switch from bytea to large objects, but in the near
term I think that's your only option if the dump file size is a
showstopper problem for you.

regards, tom lane

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Re: Slow pg_dump

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Thanks for the info on TOAST. We're still finding our legs with =
Postgres after several years on MySQL.

We do have the flexibility to adjust our data types and schema if we =
need to. We try to keep it to a minimum, but it's doable. I'm =
completely open to the possibility that we just have a very inefficient =
DB design or that we're misusing the data types.

We'll be running some more tests looking for the sweet spot between time =
and size. I expect we'll find a good balance somewhere.

Thanks!

-----Original Message-----
From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
Sent: Mon 4/14/2008 7:58 PM
To: Phillip Smith
Cc: Ryan Wells; pgsql-admin@postgresql.org
Subject: Re: [ADMIN] Slow pg_dump=20
=20
"Phillip Smith" writes:
>> Here's my interpretation of those results: the TOAST tables for
>> our image files are compressed by Postgres. During the backup,
>> pg_dump uncompresses them, and if compression is turned on,
>> recompresses the backup. Please correct me if I'm wrong there.

No, the TOAST tables aren't compressed, they're pretty much going to be
the raw image data (plus a bit of overhead).

What I think is happening is that COPY OUT is encoding the bytea
data fairly inefficiently (one byte could go to \\nnn, five bytes)
and the compression on the pg_dump side isn't doing very well at buying
that back.

I experimented a bit and noticed that pg_dump -Fc is a great deal
smarter about storing large objects than big bytea fields --- it seems
to be pretty nearly one-to-one with the original data size when storing
a compressed file that was put into a large object. I dunno if it's
practical for you to switch from bytea to large objects, but in the near
term I think that's your only option if the dump file size is a
showstopper problem for you.

regards, tom lane


------_=_NextPart_001_01C89E95.39EFC9A8
Content-Type: text/html;
charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable




charset=3Diso-8859-1">
6.5.7652.24">

Thanks for the info on TOAST.  We're still =
finding our legs with Postgres after several years on MySQL.



We do have the flexibility to adjust our data types and schema if we =
need to.  We try to keep it to a minimum, but it's doable.  =
I'm completely open to the possibility that we just have a very =
inefficient DB design or that we're misusing the data types.



We'll be running some more tests looking for the sweet spot between time =
and size.  I expect we'll find a good balance somewhere.



Thanks!



-----Original Message-----

From: Tom Lane [ HREF=3D"mailto:tgl@sss.pgh.pa.us">mailto:tgl@sss.pgh.pa.us]

Sent: Mon 4/14/2008 7:58 PM

To: Phillip Smith

Cc: Ryan Wells; pgsql-admin@postgresql.org

Subject: Re: [ADMIN] Slow pg_dump



"Phillip Smith" <phillip.smith@weatherbeeta.com.au> =
writes:

>> Here's my interpretation of those results: the TOAST tables =
for

>> our image files are compressed by Postgres.  During the =
backup,

>> pg_dump uncompresses them, and if compression is turned on,

>> recompresses the backup.  Please correct me if I'm wrong =
there.



No, the TOAST tables aren't compressed, they're pretty much going to =
be

the raw image data (plus a bit of overhead).



What I think is happening is that COPY OUT is encoding the bytea

data fairly inefficiently (one byte could go to \\nnn, five bytes)

and the compression on the pg_dump side isn't doing very well at =
buying

that back.



I experimented a bit and noticed that pg_dump -Fc is a great deal

smarter about storing large objects than big bytea fields --- it =
seems

to be pretty nearly one-to-one with the original data size when =
storing

a compressed file that was put into a large object.  I dunno if =
it's

practical for you to switch from bytea to large objects, but in the =
near

term I think that's your only option if the dump file size is a

showstopper problem for you.



        =
        =
        regards, tom lane