Re: What to do about snapshot-aware defrag

[Josef Bacik <jbacik@fb.com>]

On 05/30/2014 06:00 PM, Martin wrote:
> OK... I'll jump in...
>
> On 30/05/14 21:43, Josef Bacik wrote:
>> Hello,
>>
>> TL;DR: I want to only do snapshot-aware defrag on inodes in snapshots
>> that haven't changed since the snapshot was taken.  Yay or nay (with a
>> reason why for nay)
>
> [...]
>>
>> === Summary and what I need ===
>>
>> Option 1: Only relink inodes that haven't changed since the snapshot was
>> taken.
>>
>> Pros:
>> -Faster
>> -Simpler
>> -Less duplicated code, uses existing functions for tricky operations so
>> less likely to introduce weird bugs.
>>
>> Cons:
>> -Could possibly lost some of the snapshot-awareness of the defrag.  If
>> you just touch a file we would not do the relinking and you'd end up
>> with twice the space usage.
> [...]
>
>
> Obvious way to go for fast KISS.
>
>
> One question:
>
> Will option one mean that we always need to mount with noatime or
> read-only to allow snapshot defragging to do anything?
>

Yeah atime would screw this up, I hadn't thought of that.  With that 
being the case I think the only option is to keep the old behavior, we 
don't want to screw up stuff like this just because users used a backup 
program on their snapshot and didn't use noatime.  Thanks,

Josef