Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

On Tue, Aug 25, 2020 at 9:31 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Aug 24, 2020 at 9:41 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Sat, Aug 22, 2020 at 8:38 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Aug 21, 2020 at 5:10 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > > I have reviewed and tested the patch and the changes look fine to me.
> > > >
> > >
> > > Thanks, I will push the next patch early next week (by Tuesday) unless
> > > you or someone else has any more comments on it. The summary of the
> > > patch (v52-0001-Extend-the-BufFile-interface, attached with my
> > > previous email) I am planning to push is: "It extends the BufFile
> > > interface to support temporary files that can be used by the single
> > > backend when the corresponding files need to be survived across the
> > > transaction and need to be opened and closed multiple times. Such
> > > files need to be created as a member of a SharedFileSet. We have
> > > implemented the interface for BufFileTruncate to allow files to be
> > > truncated up to a particular offset and extended the BufFileSeek API
> > > to support SEEK_END case. We have also added an option to provide a
> > > mode while opening the shared BufFiles instead of always opening in
> > > read-only mode. These enhancements in BufFile interface are required
> > > for the upcoming patch to allow the replication apply worker, to
> > > properly handle streamed in-progress transactions."
> >
> > While reviewing 0002, I realized that instead of using individual
> > shared fileset for each transaction, we can use just one common shared
> > file set. We can create individual buffile under one shared fileset
> > and whenever a transaction commits/aborts we can just delete its
> > buffile and the shared fileset can stay.
> >
>
> I think the existing design is superior as it allows the flexibility
> to create transaction files in different temp_tablespaces which is
> quite important to consider as we know the files will be created only
> for large transactions. Once we fix the sharedfileset for a worker all
> the files will be created in the temp_tablespaces chosen for the first
> time apply worker creates it even if it got changed at some later
> point of time (user can change its value and then do reload config
> which I think will impact the worker settings as well). This all can
> happen because we set the tablespaces at the time of
> SharedFileSetInit.

Yeah, I agree with this point, that if we use the single shared
fileset then it will always use the same tablespace for all the
streaming transactions. And, we might get the benefit of concurrent
I/O if we use different tablespaces as we are not immediately flushing
the files to the disk.

> The other relatively smaller thing which I don't like is that we
> always need to create a buffile for subxact even though we don't need
> it. We might be able to find some solution for this but I guess the
> previous point is what bothers me more.

Yeah, if we go this way we might need to find some solution to this.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com