Re: speed up cp --reflink=always

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

On 10/16/2016 09:48 PM, Hans van Kranenburg wrote:
> On 10/16/2016 08:54 PM, Stefan Priebe - Profihost AG wrote:
>> Am 16.10.2016 um 00:37 schrieb Hans van Kranenburg:
>>> On 10/15/2016 10:49 PM, Stefan Priebe - Profihost AG wrote:
>>>>
>>>> cp --reflink=always takes sometimes very long. (i.e. 25-35 minutes)
>>>>
>>>> An example:
>>>>
>>>> source file:
>>>> # ls -la vm-279-disk-1.img
>>>> -rw-r--r-- 1 root root 204010946560 Oct 14 12:15 vm-279-disk-1.img
>>>>
>>>> target file after around 10 minutes:
>>>> # ls -la vm-279-disk-1.img.tmp
>>>> -rw-r--r-- 1 root root 65022328832 Oct 15 22:13 vm-279-disk-1.img.tmp
>>>
>>> Two quick thoughts:
>>> 1. How many extents does this img have?
>>
>> filefrag says:
>> 1011508 extents found
> 
> To cp --reflink this, the filesystem needs to create a million new
> EXTENT_DATA objects for the new file, which point all parts of the new
> file to all the little same parts of the old file, and probably also
> needs to update a million EXTENT_DATA objects in the btrees to add a
> second backreference back to the new file.

Ehm, the second one is EXTENT_ITEM, not EXTENT_DATA.

>>> 2. Is this an XY problem? Why not just put the img in a subvolume and
>>> snapshot that?
>>
>> Sorry what's XY problem?
> 
> It means that I suspected that your actual goal is not spending time to
> work on optimizing how cp --reflink works, but that you just want to use
> the quickest way to have a clone of the file.
> 
> An XY problem is when someone has problem X, then thinks about solution
> Y to solve it, then runs into a problem/limitation/whatever when trying
> Y and asks help with that actual problem when doing Y while there might
> in the end be a better solution to get X done.
> 
>> Implementing cp reflink was easier - as the original code was based on
>> XFS. But shouldn't be cp reflink / clone a file be nearly identical to a
>> snapshot? Just creating refs to the extents?
> 
> Snapshotting a subvolume only has to write a cowed copy of the top-level
> information of the subvolume filesystem tree, and leaves the extent tree
> alone. It doesn't have to do 2 million different things. \o/
> 


-- 
Hans van Kranenburg