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What is WAL used for?

I'm just trying to figure out the terminology that is used on this board and
wanted to know what is WAL and what roll does it play in Postgresql?

Thanks
Nov 12 '05
13 9199
On Fri, 2003-12-05 at 02:40, Thierry Missimilly wrote:
With the data=writeback mode, I increase the TPS by 18% and dicrease the wait
I/O from 54% to 30%.
I did not change my filesystem to ext2 as I have to have to cancel the partition
and recreate all the database. Futhermore, i have understood that journaled
filesystem allowed better and faster fsck after a Power off crash and it is not
redundant with the WAL Crash recovery.
I think that "journaling " is at file system level and WAL is above in the
Database level. What happen if the xlog filesystem has been breakdown by a power
off. All the Data concisentcy done by PG will be lost. I hope that data stored
in the FS journal, can avoid that.


What's the recommended method of changing an ext3 partition to
data=writeback mode on RH9?

I tried the tune2fs -j method to set the default journal type and
rebooted, but saw *no* performance differences, so I was wondering if
setting the default actually put it in writeback mode.

Does anyone know if there's an easy way to verify the mode, or if I'm
setting it wrong ?


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Nov 12 '05 #11


Cott Lang wrote:
On Fri, 2003-12-05 at 02:40, Thierry Missimilly wrote:
With the data=writeback mode, I increase the TPS by 18% and dicrease the wait
I/O from 54% to 30%.
I did not change my filesystem to ext2 as I have to have to cancel the partition
and recreate all the database. Futhermore, i have understood that journaled
filesystem allowed better and faster fsck after a Power off crash and it is not
redundant with the WAL Crash recovery.
I think that "journaling " is at file system level and WAL is above in the
Database level. What happen if the xlog filesystem has been breakdown by a power
off. All the Data concisentcy done by PG will be lost. I hope that data stored
in the FS journal, can avoid that.
What's the recommended method of changing an ext3 partition to
data=writeback mode on RH9?


For exemple, with the root privilege :
mount -t ext3 -o data=writeback /dev/sdb1 /data1

or in /etc/fstab :
/dev/sdb1 /data1 ext3 data=writeback 0 0


I tried the tune2fs -j method to set the default journal type and
rebooted, but saw *no* performance differences, so I was wondering if
setting the default actually put it in writeback mode.

Does anyone know if there's an easy way to verify the mode, or if I'm
setting it wrong ?

By default the mode is "ordered".
The command : mount returns how FS are mounted.

Thierry Missimilly

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Nov 12 '05 #12
On Mon, 2003-12-08 at 02:53, Thierry Missimilly wrote:
For exemple, with the root privilege :
mount -t ext3 -o data=writeback /dev/sdb1 /data1


Thanks, but my problem is I need to change the root partition to
data=writeback, which you can't do by changing fstab. :(


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Nov 12 '05 #13
Cott Lang <co**@internets taff.com> writes:
On Mon, 2003-12-08 at 02:53, Thierry Missimilly wrote:
For exemple, with the root privilege :
mount -t ext3 -o data=writeback /dev/sdb1 /data1


Thanks, but my problem is I need to change the root partition to
data=writeback, which you can't do by changing fstab. :(


I think there is a kernel boot argument for this, but I don't know
what it's called. Google will probably turn it up...

You shouldn't have your database files on / on a production system
anyway. /var should be a separate partition.

-Doug

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Nov 12 '05 #14

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