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

On Wed, Aug 19, 2020 at 1:35 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Aug 19, 2020 at 12:20 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Wed, Aug 19, 2020 at 10:10 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Aug 17, 2020 at 6:29 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > >
> > > >
> > > > In last patch v49-0001, there is one issue, Basically, I have called
> > > > BufFileFlush in all the cases. But, ideally, we can not call this if
> > > > the underlying files are deleted/truncated because those files/blocks
> > > > might not exist now. So I think if the truncate position is within
> > > > the same buffer we just need to adjust the buffer, otherwise we just
> > > > need to set the currFile and currOffset to the absolute number and set
> > > > the pos and nbytes 0. Attached patch fixes this issue.
> > > >
> > >
> > > Few comments on the latest patch v50-0001-Extend-the-BufFile-interface
> > > 1.
> > > +
> > > + /*
> > > + * If the truncate point is within existing buffer then we can just
> > > + * adjust pos-within-buffer, without flushing buffer. Otherwise,
> > > + * we don't need to do anything because we have already deleted/truncated
> > > + * the underlying files.
> > > + */
> > > + if (curFile == file->curFile &&
> > > + curOffset >= file->curOffset &&
> > > + curOffset <= file->curOffset + file->nbytes)
> > > + {
> > > + file->pos = (int) (curOffset - file->curOffset);
> > > + return;
> > > + }
> > >
> > > I think in this case you have set the position correctly but what
> > > about file->nbytes? In BufFileSeek, it was okay not to update 'nbytes'
> > > because the contents of the buffer are still valid but I don't think
> > > the same is true here.
> > >
> >
> > I think you need to set 'nbytes' to curOffset as per your current
> > patch as that is the new size of the file.
> > --- a/src/backend/storage/file/buffile.c
> > +++ b/src/backend/storage/file/buffile.c
> > @@ -912,6 +912,7 @@ BufFileTruncateShared(BufFile *file, int fileno,
> > off_t offset)
> > curOffset <= file->curOffset + file->nbytes)
> > {
> > file->pos = (int) (curOffset - file->curOffset);
> > + file->nbytes = (int) curOffset;
> > return;
> > }
> >
> > Also, what about file 'numFiles', that can also change due to the
> > removal of certain files, shouldn't that be also set in this case
>
> Right, we need to set the numFile. I will fix this as well.

I think there are a couple of more problems in the truncate APIs,
basically, if the curFile and curOffset are already smaller than the
truncate location the truncate should not change that. So the
truncate should only change the curFile and curOffset if it is
truncating the part of the file where the curFile or curOffset is
pointing. I will work on those along with your other comments and
submit the updated patch.

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