Re: Pre-allocating WAL files

On 12/7/21, 9:35 AM, "Bossart, Nathan" <bossartn@amazon.com> wrote:
> On 12/7/21, 12:29 AM, "Bharath Rupireddy" <bharath.rupireddyforpostgres@gmail.com> wrote:
>> Why can't the walwriter pre-allocate some of the WAL segments instead
>> of a new background process? Of course, it might delay the main
>> functionality of the walwriter i.e. flush and sync the WAL files, but
>> having checkpointer do the pre-allocation makes it do another extra
>> task. Here the amount of walwriter work vs checkpointer work, I'm not
>> sure which one does more work compared to the other.
>
> The argument against adding it to the WAL writer is that we want it to
> run with low latency to flush asynchronous commits.  If we added WAL
> pre-allocation to the WAL writer, there could periodically be large
> delays.

To your point on trying to avoid giving the checkpointer extra tasks
(basically what we are talking about on the other thread [0]), WAL
pre-allocation might not be of much concern because it will generally
be a small, fixed (and configurable) amount of work, and it can be
performed concurrently with the checkpoint.  Plus, WAL pre-allocation
should ordinarily be phased out as WAL segments become eligible for
recycling.  IMO it's not comparable to tasks like
CheckPointSnapBuild() that can delay checkpointing for a long time.

Nathan

[0] https://www.postgresql.org/message-id/flat/C1EE64B0-D4DB-40F3-98C8-0CED324D34CB%40amazon.com