Re: Query about proposed dedup patches and behaviours

[Duncan <1i5t5.duncan@cox.net>]

Austin S. Hemmelgarn posted on Thu, 14 Jan 2016 14:41:27 -0500 as
excerpted:

> On 2016-01-14 14:26, Liu Bo wrote:
>> On Thu, Jan 14, 2016 at 11:46:33AM -0500, Austin S. Hemmelgarn wrote:
>>> On 2016-01-14 11:13, James Hogarth wrote:

>>>> Finally what's the present situation with regards to defragmentation
>>>> and deduplication? Is it safe to turn on autodefrag now when using
>>>> snapshots and duperemove? What should the behaviour be with the
>>>> proposed 4.5 dedup patches if both inline dedup and autodefrag are
>>>> enabled as mount options?

>>> I'm not entirely certain how deduplication would interact with any
>>> form of defragmentation.  I'm pretty certain though that autodefrag
>>> does properly handle snapshots, such that the reflinks aren't broken,
>>> and it's the original copy that gets any shared extents defragmented
>>> into it.
>>
>> If it refers to snapshot-aware defrag, it's been disabled, so now btrfs
>> will not maintain reflinks between snapshots.
>>
> I was under the impression that autodefrag had been done separately from
> the snapshot-aware manually triggered defrag, and that it's always been
> snapshot aware.

Hugo should really explain as he was the one that said that, but upon 
looking into it, he found that while he was correct in a sense, his 
reasoning was a bit narrow, and autodefrag isn't snapshot aware in the 
wider context.

Without attempting to explain his reasoning as I think I sort of 
understand it but not well enough to try to explain, autodefrag isn't 
snapshot aware and will break reflinks, but due to $reasons, autodefrag's 
damage to reflinking apparently isn't as bad as manual defrag.

That's the best I can do to explain the situation.  In general, 
autodefrag remains bad for reflinks, but apparently not h***-bad, as 
manual defrag is.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman