Re: agcount for 2TB, 4TB and 8TB drives

[Dave Chinner <david@fromorbit.com>]

On Mon, Oct 16, 2017 at 01:00:32PM +0300, Avi Kivity wrote:
> On 10/16/2017 01:00 AM, Dave Chinner wrote:
> >On Sun, Oct 15, 2017 at 12:36:03PM +0300, Avi Kivity wrote:
> >>On 10/15/2017 01:42 AM, Dave Chinner wrote:
> >>>On Fri, Oct 13, 2017 at 11:13:24AM +0300, Avi Kivity wrote:
> >>Even better would be if XFS would detect the sequential write and
> >>start allocating ahead of it.
> >That's what delayed allocation does with buffered IO. We
> >specifically do not do that with direct IO because it's direct IO
> >and we only do exactly what the IO the user submits requires us to
> >do.
> >
> >As it is, I'm not sure that it would gain us anything over extent
> >size hints because they are effectively doing exactly the same thing
> >(i.e.  allocate ahead) on every write that hits a hole beyond
> >EOF when extending the file....
> 
> If I understand correctly, you do get momentary serialization when
> you cross a hint boundary, while with allocate ahead, you would not.

Allocate ahead still requires a threshold to be crossed to trigger
allocation. So it doesn't get rid of allocation, it just changes
what IO triggers it.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com