Re: btrfs and swap files on SSD's ?

[Chris Mason <chris.mason@oracle.com>]

On Tue, 2009-01-20 at 09:35 -0700, Anthony Roberts wrote:
> > The second is an implementation detail of the linux swap file code.  It
> > expects filesystems don't move blocks around, and takes a mapping of the
> > blocks in the FS once.
> > 
> > This doesn't work with btrfs because we do move blocks around all the
> > time.
> 
> That's interesting. I have a few questions:
> 
> -Is creating a loopback device from the file any different, or does that
> lead to the same problems?

The loopback device would probably work.  At least it would cover blocks
that move around.

> 
> -Would mounting a filesystem image via loopback device cause similar
> problems?

loopback goes through safer APIs.

> 
> -Would this be viable if using a dedicated nodatacow subvolume, or is that
> still too risky because of the odd case where you do cow?

nodatacow is allowed to COW  when there are snapshots or clones.  I
wouldn't recommend swapfiles on it just because people can easily forget
about the swapfile restriction.

I plan on sending a patch to at least disable swapfiles for btrfs in
this kernel cycle.  Later on we can work out the swap bmapping apis with
the VM maintainers.

> 
> -Does online defragmentation hurt this as well?
> 

Yes.  Online defrag in XFS may have problems with this too, I'm asking
the xfs people if they have worked around this.

-chris