hole punching performance

[Dave Chinner]

On Sun, Jan 13, 2013 at 01:49:41PM -0600, Ben Myers wrote:
> On Sun, Jan 13, 2013 at 11:36:27AM -0500, Bradley C. Kuszmaul wrote:
> > On Sun, Jan 13, 2013 at 4:26 AM, Florian Weimer <fw at deneb.enyo.de> wrote:
> > > I once was a heavy Berkeley DB user and had files with dozens of gigabytes
> > > containing hundreds of thousands of extents, and open() times in the order
> > > of minutes were not unusual with a cold cache and other concurrent read
> > > activities from the same RAID device.
> 
> Ouch.  I would not have expected open times that bad.

That will only happen if the extent list needs to be read during the
open() call. That will only occur if O_TRUNC is set. Otherwise, the
extents are read on the first syscall that needs them to be read in
(read, write, alloc, punch, truncate, etc) and that's when the
latency occurs.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com