Re: Improve lseek scalability v3

On Mon, Sep 19, 2011 at 08:31:00AM -0400, Stephen Frost wrote:
> * Benjamin LaHaise (bcrl(at)kvack(dot)org) wrote:
> > For such tables, can't Postgres track the size of the file internally? I'm
> > assuming it's keeping file descriptors open on the tables it manages, in
> > which case when it writes to a file to extend it, the internally stored size
> > could be updated. Not making a syscall at all would scale far better than
> > even a modified lseek() will perform.
>
> We'd have to have it in shared memory and have a lock around it, it
> wouldn't be cheap at all.

Yep, that makes perfect sense. After all, the kernel does basically the
same thing to maintain this information; why should we have userspace
duplicating the same infrastructure?

I must admit, I'd never heard of this usage of lseek to get the current
size of a file before; I'd assumed everybody used fstat. Given this
legitimate reason for a high-frequency calling of lseek, I withdraw my
earlier objection to the patch series.

--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."