hole punching performance

[Florian Weimer]

* 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.

Indeed, I think I have misremembered.  The delay occurred when
Berkeley DB tried to read the first page from disk.