Thread: ext3 journalling type
The ext3fs allows to selet type of journalling to be used with
filesystem. Journalling pretty much "mirrors" the work of WAL
logging by PostgreSQL... I wonder which type of journalling
is best for PgSQL in terms of performance.
Choices include:
journal
All data is committed into the journal prior to being
written into the main file system.
ordered
This is the default mode. All data is forced directly
out to the main file system prior to its metadata being
committed to the journal.
writeback
Data ordering is not preserved - data may be written into
the main file system after its metadata has been commit-
ted to the journal. This is rumoured to be the highest-
throughput option. It guarantees internal file system
integrity, however it can allow old data to appear in
files after a crash and journal recovery.
Am I right to assume that "writeback" is both fastest and at the same
time as safe to use as ordered? Maybe any of you did some benchmarks?
Regards,
Dawid
Dawid Kuroczko wrote: > The ext3fs allows to selet type of journalling to be used with > filesystem. Journalling pretty much "mirrors" the work of WAL > logging by PostgreSQL... I wonder which type of journalling > is best for PgSQL in terms of performance. > Choices include: > journal > All data is committed into the journal prior to being > written into the main file system. > ordered > This is the default mode. All data is forced directly > out to the main file system prior to its metadata being > committed to the journal. > writeback > Data ordering is not preserved - data may be written into > the main file system after its metadata has been commit- > ted to the journal. This is rumoured to be the highest- > throughput option. It guarantees internal file system > integrity, however it can allow old data to appear in > files after a crash and journal recovery. > > Am I right to assume that "writeback" is both fastest and at the same > time as safe to use as ordered? Maybe any of you did some benchmarks? Yes. I have seen benchmarks that say writeback is fastest but I don't have any numbers handy. -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073
> Am I right to assume that "writeback" is both fastest and at > the same time as safe to use as ordered? Maybe any of you > did some benchmarks? It should be fastest because it is the least overhead, and safe because postgres does it's own write-order guaranteeing through fsync(). You should also mount the FS with the 'noatime' option. But.... For some workloads, there are tests showing that 'data=journal' can be the fastest! This is because although the data is written twice (once to the journal, and then to its real location on disk) in this mode data is written _sequentially_ to the journal, and later written out to its destination, which may be at a quieter time. There's a discussion (based around 7.2) here: http://www.kerneltraffic.org/kernel-traffic/kt20020401_160.txt M
I have some data here, no detailed analyses though:
http://www.osdl.org/projects/dbt2dev/results/fs/
Mark
On Mon, Nov 08, 2004 at 01:26:09PM +0100, Dawid Kuroczko wrote:
> The ext3fs allows to selet type of journalling to be used with
> filesystem. Journalling pretty much "mirrors" the work of WAL
> logging by PostgreSQL... I wonder which type of journalling
> is best for PgSQL in terms of performance.
> Choices include:
> journal
> All data is committed into the journal prior to being
> written into the main file system.
> ordered
> This is the default mode. All data is forced directly
> out to the main file system prior to its metadata being
> committed to the journal.
> writeback
> Data ordering is not preserved - data may be written into
> the main file system after its metadata has been commit-
> ted to the journal. This is rumoured to be the highest-
> throughput option. It guarantees internal file system
> integrity, however it can allow old data to appear in
> files after a crash and journal recovery.
>
> Am I right to assume that "writeback" is both fastest and at the same
> time as safe to use as ordered? Maybe any of you did some benchmarks?
>
> Regards,
> Dawid
>
Matt, > It should be fastest because it is the least overhead, and safe because > postgres does it's own write-order guaranteeing through fsync(). You > should also mount the FS with the 'noatime' option. This, of course, assumes that PostgreSQL is the only thing on the partition. Which is a good idea in general, but not to be taken for granted ... -- Josh Berkus Aglio Database Solutions San Francisco