Re: Use read streams in CREATE DATABASE command when the strategy is wal_log

On Tue, Jul 16, 2024 at 02:11:20PM +0300, Nazir Bilal Yavuz wrote:
> On Fri, 12 Jul 2024 at 02:52, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > On Tue, Apr 16, 2024 at 02:12:19PM +0300, Nazir Bilal Yavuz wrote:
> > > --- a/src/backend/storage/aio/read_stream.c
> > > +++ b/src/backend/storage/aio/read_stream.c
> > > @@ -549,7 +549,7 @@ read_stream_begin_relation(int flags,
> > > {
> > > stream->ios[i].op.rel = rel;
> > > stream->ios[i].op.smgr = RelationGetSmgr(rel);
> > > - stream->ios[i].op.smgr_persistence = 0;
> > > + stream->ios[i].op.smgr_persistence = rel->rd_rel->relpersistence;
> >
> > Does the following comment in ReadBuffersOperation need an update?
> >
> > /*
> > * The following members should be set by the caller. If only smgr is
> > * provided without rel, then smgr_persistence can be set to override the
> > * default assumption of RELPERSISTENCE_PERMANENT.
> > */
>
> I believe it does not need to be updated but I renamed
> 'ReadBuffersOperation.smgr_persistence' as
> 'ReadBuffersOperation.persistence'. So, this comment is updated as
> well. I think that rename suits better because persistence does not
> need to come from smgr, it could come from relation, too. Do you think
> it is a good idea? If it is, does it need a separate commit?

The rename is good. I think the comment implies "persistence" is unused when
rel!=NULL. That implication is true before the patch but false after the
patch.

> > > @@ -4667,19 +4698,31 @@ RelationCopyStorageUsingBuffer(RelFileLocator srclocator,
> >
> > > /* Iterate over each block of the source relation file. */
> > > for (blkno = 0; blkno < nblocks; blkno++)
> > > {
> > > CHECK_FOR_INTERRUPTS();
> > >
> > > /* Read block from source relation. */
> > > - srcBuf = ReadBufferWithoutRelcache(srclocator, forkNum, blkno,
> > > - RBM_NORMAL, bstrategy_src,
> > > - permanent);
> > > + srcBuf = read_stream_next_buffer(src_stream, NULL);
> > > LockBuffer(srcBuf, BUFFER_LOCK_SHARE);
> >
> > I think this should check for read_stream_next_buffer() returning
> > InvalidBuffer. pg_prewarm doesn't, but the other callers do, and I think the
> > other callers are a better model. LockBuffer() doesn't check the
> > InvalidBuffer case, so let's avoid the style of using a
> > read_stream_next_buffer() return value without checking.
>
> There is an assert in the LockBuffer which checks for the
> InvalidBuffer. If that is not enough, we may add an if check for
> InvalidBuffer but what should we do in this case? It should not
> happen, so erroring out may be a good idea.

I like this style from read_stream_reset():

while ((buffer = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
{
...
}

That is, don't iterate over block numbers. Drain the stream until empty. If
the stream returns a number of blocks higher or lower than we expected, we
won't detect that, and that's okay. It's not a strong preference, so I'm open
to arguments against that from you or others. A counterargument could be that
read_stream_reset() doesn't know the buffer count, so it has no choice. The
counterargument could say that callers knowing the block count should use the
pg_prewarm() style, and others should use the read_stream_reset() style.