Re: .ready and .done files considered harmful

On Tue, May 4, 2021 at 11:54 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> I agree that if we continue to archive one file using the archive
> command then Robert's solution of checking the existence of the next
> WAL segment (N+1) has an advantage.  But, currently, if you notice
> pgarch_readyXlog always consider any history file as the oldest file
> but that will not be true if we try to predict the next WAL segment
> name.  For example, if we have archived 000000010000000000000004 then
> next we will look for 000000010000000000000005 but after generating
> segment 000000010000000000000005, if there is a timeline switch then
> we will have the below files in the archive status
> (000000010000000000000005.ready, 00000002.history file).  Now, the
> existing archiver will archive 00000002.history first whereas our code
> will archive 000000010000000000000005 first.  Said that I don't see
> any problem with that because before archiving any segment file from
> TL 2 we will definitely archive the 00000002.history file because we
> will not find the 000000010000000000000006.ready and we will scan the
> full directory and now we will find 00000002.history as oldest file.

OK, that makes sense and is good to know.

> > > > However, that's still pretty wasteful. Every time we have to wait for
> > > > the next file to be ready for archiving, we'll basically fall back to
> > > > repeatedly scanning the whole directory, waiting for it to show up.
>
> Is this true? that only when we have to wait for the next file to be
> ready we got for scanning?  If I read the code in
> "pgarch_ArchiverCopyLoop", for every single file to achieve it is
> calling "pgarch_readyXlog", wherein it scans the directory every time.
> So I did not understand your point that only when it needs to wait for
> the next .ready file it need to scan the full directory.  It appeared
> it always scans the full directory after archiving each WAL segment.
> What am I missing?

It's not true now, but my proposal would make it true.

-- 
Robert Haas
EDB: http://www.enterprisedb.com