Re: BTRFS as image store for KVM?

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-09-25 08:48, Rich Freeman wrote:
> On Sat, Sep 19, 2015 at 9:26 PM, Jim Salter <jim@jrs-s.net> wrote:
>>
>> ZFS, by contrast, works like absolute gangbusters for KVM image storage.
>
> I'd be interested in what allows ZFS to handle KVM image storage well,
> and whether this could be implemented in btrfs.  I'd think that the
> fragmentation issues would potentially apply to any COW filesystem,
> and if ZFS has a solution for this then it would probably benefit
> btrfs to implement the same solution, and not just for VM images.
That may be tough to do however, the internal design of ZFS is _very_ 
different from that of BTRFS (and for that matter, every other 
filesystem on Linux).  Part of it may just be better data locality (if 
all of the fragments of a file are close to each other, then the 
fragmentation of the file is not as much of a performance hit), and part 
of it is probably how they do caching (and I personally _do not_ want 
BTRFS to try to do caching the way ZFS does, we have a unified pagecache 
in the VFS for a reason, we should be improving that, not trying to come 
up with multiple independent solutions).


Even aside from that however, just saying that ZFS works great for some 
particular use case isn't giving enough info, it has so many optional 
features and configuration knobs, you really need to give specifics on 
how you have ZFS set up in that case.


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