Re: btrfs-cleaner / snapshot performance analysis

[Hans van Kranenburg <hans.van.kranenburg@mendix.com>]

On 02/12/2018 03:45 PM, Ellis H. Wilson III wrote:
> On 02/11/2018 01:03 PM, Hans van Kranenburg wrote:
>>> 3. I need to look at the code to understand the interplay between
>>> qgroups, snapshots, and foreground I/O performance as there isn't
>>> existing architecture documentation to point me to that covers this
>>
>> Well, the excellent write-up of Qu this morning shows some explanation
>> from the design point of view.
> 
> Sorry, I may have missed this email.  Or perhaps you are referring to a
> wiki or blog post of some kind I'm not following actively?  Either way,
> if you can forward me the link, I'd greatly appreciate it.

You are in the To: of it:

https://www.spinics.net/lists/linux-btrfs/msg74737.html

>> nocow only keeps the cows on a distance as long as you don't start
>> snapshotting (or cp --reflink) those files... If you take a snapshot,
>> then you force btrfs to keep the data around that is referenced by the
>> snapshot. So, that means that every next write will be cowed once again,
>> moo, so small writes will be redirected to a new location, causing
>> fragmentation again. The second and third write can go in the same (new)
>> location of the first new write, but as soon as you snapshot again, this
>> happens again.
> 
> Ah, very interesting.  Thank you for clarifying!

-- 
Hans van Kranenburg