how to measure wal_buffer usage

Lonni J Friedman wrote:

After reading this interesting article on shared_buffers and wal_buffers:
http://rhaas.blogspot.com/2012/03/tuning-sharedbuffers-and-walbuffers.html

it got me wondering if my settings were ideal. Is there some way to
measure wal_buffer usage in real time, so that I could simply monitor
it for some period of time, and then come up with a way of determining
if the current setting is sufficient?

I tried googling, but every reference that I've found simply defaults
to the "trial & error" approach to performance tuning.

You can use the contrib module pg_buffercache to inspect the shared buffers.
If almost all your shared buffers have high use count (4 or 5),
shared_buffers may be too small. If not, consider reducing shared_buffers.

It's probably better to start with a moderate value and tune upwards.

You can also look at pg_statio_all_tables and pg_statio_all_indexes and
calculate the buffer hit ratio. If that is low, that's also an indication
that shared_buffers is too small.

You should distinguish between tables and indexes:
it is usually more important that indexes are cached.

Try to observe these things over time, for example by taking
snapshots every n minutes and storing the results in a table.

Yours,
Laurenz Albe

On Fri, Mar 16, 2012 at 2:45 AM, Albe Laurenz <laurenz.albe@wien.gv.at> wrote:

Lonni J Friedman wrote:

After reading this interesting article on shared_buffers and wal_buffers:
http://rhaas.blogspot.com/2012/03/tuning-sharedbuffers-and-walbuffers.html

it got me wondering if my settings were ideal.  Is there some way to
measure wal_buffer usage in real time, so that I could simply monitor
it for some period of time, and then come up with a way of determining
if the current setting is sufficient?

I tried googling, but every reference that I've found simply defaults
to the "trial & error" approach to performance tuning.

You can use the contrib module pg_buffercache to inspect the shared buffers.
If almost all your shared buffers have high use count (4 or 5),
shared_buffers may be too small.  If not, consider reducing shared_buffers.

pg_buffercache only reports on the buffer_cache, it does not report
any data on the wal_cache.

It's probably better to start with a moderate value and tune upwards.

You can also look at pg_statio_all_tables and pg_statio_all_indexes and
calculate the buffer hit ratio.  If that is low, that's also an indication
that shared_buffers is too small.

Isn't this also specific to the buffer_cache rather than the wal_cache?

You should distinguish between tables and indexes:
it is usually more important that indexes are cached.

Try to observe these things over time, for example by taking
snapshots every n minutes and storing the results in a table.

Yours,
Laurenz Albe

--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
L. Friedman                                    netllama@gmail.com
LlamaLand                       https://netllama.linux-sxs.org

Lonni J Friedman wrote:

After reading this interesting article on shared_buffers and wal_buffers:
http://rhaas.blogspot.com/2012/03/tuning-sharedbuffers-and-walbuffers.html

it got me wondering if my settings were ideal.  Is there some way to
measure wal_buffer usage in real time, so that I could simply monitor
it for some period of time, and then come up with a way of determining
if the current setting is sufficient?

I tried googling, but every reference that I've found simply defaults
to the "trial & error" approach to performance tuning.

You can use the contrib module pg_buffercache to inspect the shared buffers.
If almost all your shared buffers have high use count (4 or 5),
shared_buffers may be too small.  If not, consider reducing shared_buffers.

pg_buffercache only reports on the buffer_cache, it does not report
any data on the wal_cache.

You are right.

It's probably better to start with a moderate value and tune upwards.

You can also look at pg_statio_all_tables and pg_statio_all_indexes and
calculate the buffer hit ratio.  If that is low, that's also an indication
that shared_buffers is too small.

Isn't this also specific to the buffer_cache rather than the wal_cache?

Correct.

I don't know how to inspect usage WAL cache usage.

Yours,
Laurenz Albe