Re: Compress ReorderBuffer spill files using LZ4

On Thu, Jun 6, 2024 at 6:22 PM Julien Tachoires <julmon@gmail.com> wrote:
>
> Le jeu. 6 juin 2024 à 04:13, Amit Kapila <amit.kapila16@gmail.com> a écrit :
> >
> > On Thu, Jun 6, 2024 at 4:28 PM Julien Tachoires <julmon@gmail.com> wrote:
> > >
> > > When the content of a large transaction (size exceeding
> > > logical_decoding_work_mem) and its sub-transactions has to be
> > > reordered during logical decoding, then, all the changes are written
> > > on disk in temporary files located in pg_replslot/<slot_name>.
> > > Decoding very large transactions by multiple replication slots can
> > > lead to disk space saturation and high I/O utilization.
> > >
> >
> > Why can't one use 'streaming' option to send changes to the client
> > once it reaches the configured limit of 'logical_decoding_work_mem'?
>
> That's right, setting subscription's option 'streaming' to 'on' moves
> the problem away from the publisher to the subscribers. This patch
> tries to improve the default situation when 'streaming' is set to
> 'off'.
>

Can we think of changing the default to 'parallel'? BTW, it would be
better to use 'parallel' for the 'streaming' option, if the workload
has large transactions. Is there a reason to use a default value in
this case?

> > > 2. Do we want a GUC to switch compression on/off?
> > >
> >
> > It depends on the overhead of decoding. Did you try to measure the
> > decoding overhead of decompression when reading compressed files?
>
> Quick benchmarking executed on my laptop shows 1% overhead.
>

Thanks. We probably need different types of data (say random data in
bytea column, etc.) for this.

-- 
With Regards,
Amit Kapila.