On Thu, Dec 01, 2016 at 05:48:51PM +0200, Achilleas Mantzios wrote:
> On 01/12/2016 15:55, Robert Inder wrote:
> > I'm running  Postgres9.4 in master/hot-standby mode on a few pairs of 
> > servers.
> > 
> > While recovering from A Bit Of Bother last week, I came across a
> > posting saying that pg_xlog should be on a separate partition.
> > 
> > I tried to find out more about this, by consulting the PostgresQL
> > documentation (i.e.
> > https://www.postgresql.org/docs/9.4/static/index.html )
> > But all I could find was a mention that "It is advantageous if the log
> > is located on a different disk from the main database files".
> > 
> > The questions:
> > 1. WHY is this good?  Is it (just) to stop pg_xlog filling the
> > database disk/partition?  Or are there performance implications?
> > SPECIFICALLY: my database is currently in "/", which is on SSD.  Is it
> > better to move pg_xlog to another partition on the same SSD?  Or to a
> > physical disk or SAN?
> Performance is the reason. You would benefit from moving pg_xlog to a
> different controller with its own write cache or to a different SSD with a
> write cache which is capacitor-backed. So in enterprise/server-class setups
> the above would boost the performance. Using the same SSD with a different
> partition won't give you much.

For performance, on-disk write pattern of data in pg_xlog is sequential
writes, while there will be likely random writes on the main data folder.
On top of that, moving them to a different partition gives more flexibility
in the way to tune checkpoint-related parameters using the partition space
as a constraint for retention policy and checkpoint timings.

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