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.

>> 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
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