Re: [HACKERS] Write Ahead Logging for Hash Indexes

Robert,

* Robert Haas (robertmhaas@gmail.com) wrote:
> On Wed, Mar 15, 2017 at 10:34 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > FWIW, I'm not certain that Stephen is correct to claim that we have
> > some concrete problem with sparse files.  We certainly don't *depend*
> > on sparse storage anyplace else, nor write data in a way that would be
> > likely to trigger it; but I'm not aware that we need to work hard to
> > avoid it.
>
> That theory seems inconsistent with how mdextend() works.  My
> understanding is that we zero-fill the new blocks before populating
> them with actual data precisely to avoid running out of disk space due
> to deferred allocation at the OS level.  If we don't care about
> failures due to deferred allocation at the OS level, we can rip that
> logic out and improve the performance of relation extension
> considerably.  If we do care about failures due to deferred
> allocation, then leaving holes in the file is a bad idea.

That is a fantastic point.

Thanks!

Stephen