ext3

Hello,

Gabor Szima asked us to translate the letter below.

"I read that ext3 writeback mode is recommended for PostgreSQL. I made
some tests.

data=ordered data=writeback
----------------------------------------------------------------------
restoredb: 2m16.790s 1m42.367s
UPDATE <tbl1> (17krows): 9.289s 7.147s
UPDATE <tbl1> (17krows) (2.): 10.480s 3.778s
VACUUM ANALYZE <tbl1>: 9.364s 0.986s !
VACUUM FULL <tbl1>: 16.071s 2.575s
REINDEX TABLE <tbl1>: 3.815s 1.886s
----------------------------------------------------------------------

It's seductive.
However I made some crash-tests too. Updated 4 tables simultaneously and
recurring for 10 to 120s, then powered off the machine (without the
reset button. i just pulled out the cable).

SEQ RECOVERY-WARNINGS VACUUM
-------------------------------
01: 1650 OK (WARNING: invalid page header in
block 769 of relation "18800"; zeroing out page)
02: 3 FATAL (ERROR: could not access status of
transaction 37814272)
------------------------------- (DETAIL: could not open file
"/data/pgdata/pg_clog/0024": No such file or directory)

I have stopped my tests at this point because this is not for production
use. The database was corrupted.

With ordered mode I got this:

ext3-noatime,data=ordered:

SEQ RECOVERY-WARNINGS VACUUM
------------------------------
01: 0 OK
02: 0 OK
03: 0 OK
04: 0 W,OK (relation "<tbl>" page 398 is
uninitialized --- fixing)
05: 0 OK
06: 0 OK
07: 0 W,OK (relation "<tbl>" page 911 is
uninitialized --- fixing)
08: 0 OK
09: 0 OK
10: 0 OK
------------------------------

I think that writeback mode first records the data then the inode, and
the ordered mode does it in reverse order. I also mean that postgres
log requires the inode recorded correctly, the data loss is handled by
the WAL.

AMD XP2000, 512MB RAM, PostgreSQL 7.4.6 (i686), linux-2.4.28, gcc-3.3.5,
Adaptec 29160, WD Enterprise 4360 (SCSI, SCA-80)

I made mkfs and initdb before every tests and I repeated them in reverse
order too. No quake3 ran in the background.

-Sygma"

Sorry for my english.

Mage