What is WAL used for?

[Relaxin]

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

[Jonathan Bartlett]

WAL is write-ahead logging. Basically, before the database actually
performs an operation, it writes in a log what it's about to do. Then, it
goes and does it. This ensures data consistency. Let's say that the
computer was powered off suddenly. There are several points that could
happen:

1) before a write - in this case the database would be fine with or
without write-ahead logging.

2) during a write - without write-ahead logging, if the machine is powered
off during a write, the database has no way of knowing what remained to be
written, or what was being written. WIth Postgres, this is furthere
broken down into two possibilities:

* The power-off occurred while it was writing to the log - in this
case, the log is rolled back. The database is unaffected because the data
was never written to the database proper.

* The power-off occurred after writing to the log, while writing to
disk - in this case, Postgres can simply read from the log what was
supposed to be written, and complete the write.

3) after a write - again, this does not affect Postgres either with or
without WAL.

In addition, WAL increases PostgreSQL's efficiency, because it can delay
random-access writes to disk, and just do sequential writes to the log for
a long time. This reduces the amount of head-seek the dissk are doing.
If you store your WAL files on a different disk, you get even more speed
advantages.

Jon

On Tue, 25 Nov 2003, Relaxin wrote:

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

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[Thierry Missimilly]

Jonathan,

Could you tell me what is the real impact of "fsync=fals e" on the WAL and on the
database in the same catastrophic scenario ?

Thierry Missimilly

Jonathan Bartlett wrote:

WAL is write-ahead logging. Basically, before the database actually
performs an operation, it writes in a log what it's about to do. Then, it
goes and does it. This ensures data consistency. Let's say that the
computer was powered off suddenly. There are several points that could
happen:

1) before a write - in this case the database would be fine with or
without write-ahead logging.

2) during a write - without write-ahead logging, if the machine is powered
off during a write, the database has no way of knowing what remained to be
written, or what was being written. WIth Postgres, this is furthere
broken down into two possibilities:

* The power-off occurred while it was writing to the log - in this
case, the log is rolled back. The database is unaffected because the data
was never written to the database proper.

* The power-off occurred after writing to the log, while writing to
disk - in this case, Postgres can simply read from the log what was
supposed to be written, and complete the write.

3) after a write - again, this does not affect Postgres either with or
without WAL.

In addition, WAL increases PostgreSQL's efficiency, because it can delay
random-access writes to disk, and just do sequential writes to the log for
a long time. This reduces the amount of head-seek the dissk are doing.
If you store your WAL files on a different disk, you get even more speed
advantages.

Jon

On Tue, 25 Nov 2003, Relaxin wrote:

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

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[Jonathan Bartlett]

> Could you tell me what is the real impact of "fsync=fals e" on the WAL and on the

database in the same catastrophic scenario ?
I am not certain on this point, but I believe fsync=false messes up the
whole thing. The nice thing about WAL is that fsync is no longer as much
of a slowdown, because PG rarely has to do random-access writes to the
disk.

Jon

Thierry Missimilly

Jonathan Bartlett wrote:

WAL is write-ahead logging. Basically, before the database actually
performs an operation, it writes in a log what it's about to do. Then, it
goes and does it. This ensures data consistency. Let's say that the
computer was powered off suddenly. There are several points that could
happen:

1) before a write - in this case the database would be fine with or
without write-ahead logging.

2) during a write - without write-ahead logging, if the machine is powered
off during a write, the database has no way of knowing what remained to be
written, or what was being written. WIth Postgres, this is furthere
broken down into two possibilities:

* The power-off occurred while it was writing to the log - in this
case, the log is rolled back. The database is unaffected because the data
was never written to the database proper.

* The power-off occurred after writing to the log, while writing to
disk - in this case, Postgres can simply read from the log what was
supposed to be written, and complete the write.

3) after a write - again, this does not affect Postgres either with or
without WAL.

In addition, WAL increases PostgreSQL's efficiency, because it can delay
random-access writes to disk, and just do sequential writes to the log for
a long time. This reduces the amount of head-seek the dissk are doing.
If you store your WAL files on a different disk, you get even more speed
advantages.

Jon

On Tue, 25 Nov 2003, Relaxin wrote:

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

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[Thierry Missimilly]

Jon,

I have tried a little bench with pgbench on my 2 proc 2.4 Gb with 4 GB RAM and Linux
RH 9.0.
The database size is 700 MB, so it can be loaded in memory.
Postgres 7.4 is on disk sda (Root disk)
Meta Data are on disk sdb
bench data are on disk sdc

When pgbench is running, i can see with top tool that the CPU are 53% in I/O wait. And
mainling because postgres is writting block on sdb disk. And the Transaction Per
Second (tps) are 222.

By setting "fsync=fals e", the CPU I/O wait decrease to 0.6%. And the result tps is :
466.

So, should i conclude that even if the whole database is in memory, the TPS result is
slow down by the WAL mecanism which wait for writting the log on disk ?
And the main thing to increase the TPS and preserve the consistency of data in case of
crash is to increase the I/O throughput of the Postgres WAL disk by creating RAID0 on
fiber channel subsystem (I will test that as soon asap).

Regards,
Thierry

Jonathan Bartlett wrote:

Could you tell me what is the real impact of "fsync=fals e" on the WAL and on the
database in the same catastrophic scenario ?

I am not certain on this point, but I believe fsync=false messes up the
whole thing. The nice thing about WAL is that fsync is no longer as much
of a slowdown, because PG rarely has to do random-access writes to the
disk.

Jon

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[Holger Hoffstaette]

On Fri, 28 Nov 2003 15:19:36 +0100, Thierry Missimilly wrote:

I have tried a little bench with pgbench on my 2 proc 2.4 Gb with 4 GB RAM
and Linux RH 9.0.
...
Which filesystem in which mode? Yes, that's relevant and in fact the
make-or-break factor here, at least from the POV of the hard drive.
I guess RH9 uses ext3 in journaled mode by default, which does data as
well as metadata journaling. Retry your benchmarks with both ext2 and ext3
in data=writeback mode; both results should be much closer to each other.
So, should i conclude that even if the whole database is in memory, the
TPS result is slow down by the WAL mecanism which wait for writting the

No, you need to take the working of your filesystem into account. As soon
as data journaling comes into play, it is normal and in fact unavoidable
that performance drops, because everything is written effectively twice -
once into the log, once into the file, and to do so the drive has to move.
WAL with ext3's data journaling is quite unnecessary because the WAL
sort of IS the database's journal.

Holger
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[William Yu]

Holger Hoffstaette wrote:

No, you need to take the working of your filesystem into account. As soon
as data journaling comes into play, it is normal and in fact unavoidable
that performance drops, because everything is written effectively twice -
once into the log, once into the file, and to do so the drive has to move.
WAL with ext3's data journaling is quite unnecessary because the WAL
sort of IS the database's journal.

Logically seems right but in practice may be untrue. I've found for my
apps, data=journal performs better. When I was picking filesystems, I
did a whole bunch of Googling and there were quite a few people who also
said data=journal performed faster for their Postgres or DB config.
Here's one explanation I found:

"If the database is seeking all over the filesystem and then running
fsync(), then ext3 in data=journal mode can make a huge difference,
because all the dirty data is written out *linearly* to the journal, for
later aysnchronous writeback. This can offer 10x speedups or more."

[Jonathan Bartlett]

> WAL with ext3's data journaling is quite unnecessary because the WAL

sort of IS the database's journal.
I believe you are mistaken. ext3 data journalling only does the
filesystem. It has no concept of the structure of the database itself.
WAL is still necessary to keep consistency on the table itself.

Holger
--
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[Bruce Momjian]

Jonathan Bartlett wrote:

WAL with ext3's data journaling is quite unnecessary because the WAL
sort of IS the database's journal.

I believe you are mistaken. ext3 data journalling only does the
filesystem. It has no concept of the structure of the database itself.
WAL is still necessary to keep consistency on the table itself.

What he means is that PostgreSQL doesn't need the file contents restore
pristine on crash recovery, just the directory structure and WAL can
recreate the file contents.

--
Bruce Momjian | http://candle.pha.pa.us
pg***@candle.ph a.pa.us | [phone]
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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[Thierry Missimilly]

Holger Hoffstaette wrote:

On Fri, 28 Nov 2003 15:19:36 +0100, Thierry Missimilly wrote:

I have tried a little bench with pgbench on my 2 proc 2.4 Gb with 4 GB RAM
and Linux RH 9.0.
...

Which filesystem in which mode? Yes, that's relevant and in fact the
make-or-break factor here, at least from the POV of the hard drive.
I guess RH9 uses ext3 in journaled mode by default, which does data as
well as metadata journaling. Retry your benchmarks with both ext2 and ext3
in data=writeback mode; both results should be much closer to each other.

You are right, my filesystem types are ext3.

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.

Thierry Missimilly
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