Re: defrag vs autodefrag

[Filipe Manana <fdmanana@gmail.com>]

On Mon, Dec 21, 2015 at 8:14 AM, Hugo Mills <hugo@carfax.org.uk> wrote:
> On Sun, Dec 20, 2015 at 07:26:07PM -0600, Donald Pearson wrote:
>> I read an implication in a different thread that defrag and autodefrag
>> behave differently in that autodefrag is more snapshot friendly for
>> COW data.
>>
>> Did I understand that correctly?  I have not been doing defrag on my
>> virtual machine image directory because I do use a snapshot schedule
>> and the way I understood things, a defrag would basically decouple the
>> live data from the snapshots and greatly increase utilization.
>>
>> It sounded like autodefrag does not have this problem?
>
>    Correct.

Hum?
How is that so? Snapshot-aware defrag was disabled almost 2 years ago,
and that piece of code is used both by a "manual" defrag (ioctl) and
by automatic defrag.

>
>> If that's true, is there any case where it would not be best practice
>> to mount with autodefrag enabled?
>
>    When you are already tight on I/O bandwidth for your application.
> autodefrag increases the amount of I/O the disks do (because it's
> rewriting parts of the file near each write, as well as just the piece
> that's being written by userspace).
>
>    Hugo.
>
> --
> Hugo Mills             | UNIX: Japanese brand of food containers
> hugo@... carfax.org.uk |
> http://carfax.org.uk/  |
> PGP: E2AB1DE4          |



-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."