Re: [Patch] ALTER SYSTEM READ ONLY

Hi Amul,

On Tue, Jun 16, 2020 at 6:56 AM amul sul <sulamul(at)gmail(dot)com> wrote:
> The proposed feature is built atop of super barrier mechanism commit[1] to
> coordinate
> global state changes to all active backends. Backends which executed
> ALTER SYSTEM READ { ONLY | WRITE } command places request to checkpointer
> process to change the requested WAL read/write state aka WAL prohibited and
> WAL
> permitted state respectively. When the checkpointer process sees the WAL
> prohibit
> state change request, it emits a global barrier and waits until all
> backends that
> participate in the ProcSignal absorbs it.

Why should the checkpointer have the responsibility of setting the state
of the system to read-only? Maybe this should be the postmaster's
responsibility - the checkpointer should just handle requests to
checkpoint. I think the backend requesting the read-only transition
should signal the postmaster, which in turn, will take on the aforesaid
responsibilities. The postmaster, could also additionally request a
checkpoint, using RequestCheckpoint() (if we want to support the
read-onlyness discussed in [1]). checkpointer.c should not be touched by
this feature.

Following on, any condition variable used by the backend to wait for the
ALTER SYSTEM command to finish (the patch uses
CheckpointerShmem->readonly_cv), could be housed in ProcGlobal.

Regards,
Soumyadeep (VMware)

[1] https://www.postgresql.org/message-id/CAE-ML%2B-zdWODAyWNs_Eu-siPxp_3PGbPkiSg%3DtoLeW9iS_eioA%40mail.gmail.com