hole punching performance

[Bradley C. Kuszmaul]

So does it cost a lot to read all the extents just to read the first block?
 I don't generally open a file without immediately reading some block, so
it doesn't really matter to me if there is a high cost for open() or a high
cost for the first read().

-Bradley

On Tue, Jan 15, 2013 at 4:31 PM, Florian Weimer <fw at deneb.enyo.de> wrote:

> * 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.
>
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