[ClusterLabs] Antw: [EXT] Re: OCFS2 fragmentation with snapshots

Ulrich Windl Ulrich.Windl at rz.uni-regensburg.de
Wed May 19 07:21:48 EDT 2021

>>> Andrei Borzenkov <arvidjaar at gmail.com> schrieb am 18.05.2021 um 13:05 in
<CAA91j0WgWgK=me=vjkXV9Ck00D-cHyJo7jju1gEWSSc7O6E7Mw at mail.gmail.com>:
> On Tue, May 18, 2021 at 1:52 PM Ulrich Windl
> <Ulrich.Windl at rz.uni‑regensburg.de> wrote:
>> Hi!
>> I thought using the reflink feature of OCFS2 would be just a nice way to 
> make crash‑consistent VM snapshots while they are running.
>> As it is a bit tricky to find out how much data is shared between
> I started to write an utility to examine the blocks allocated to the VM 
> backing files and snapshots.
>> Unfortunately (as it seems) OCFS2 fragments terribly under reflink 
> snapshots.
> Does it cause any real problem?
> ...

As we are running on SSDs now, probably not (I didn't measure performance),
but in the past using spinning disks that probably was a problem.
Still I think gathering multiple spread blocks is somewhat inefficient in the
block device layer.

>> So I wonder (while understanding the principle of copy‑on‑write for reflink

> snapshots):
>> Is there a way to avoid or undo the fragmentation?
> If you understand the principles you can answer it yourself. To avoid
> fragmentation you need to avoid reflink.

Well, if there is no "defragment utility", I might just copy the image to t a
new file from time to time...


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