[libvirt] [PATCH] add --nocow option to vol-create and vol-clone

[Jim Fehlig]

Chunyan Liu wrote:
>
> 2013/12/5 Daniel P. Berrange <berrange at redhat.com
> <mailto:berrange at redhat.com>>
>
>     On Thu, Dec 05, 2013 at 06:35:12PM +0800, Chunyan Liu wrote:
>     > Btrfs has terrible performance when hosting VM images, even more
>     when the guest
>     > in those VM are also using btrfs as file system. One way to
>     mitigate this bad
>     > performance is to turn off COW attributes on VM files (since
>     having copy on
>     > write for this kind of data is not useful).
>     >
>     > According to 'chattr' manpage, NOCOW could be set to new or
>     empty file only on
>     > btrfs, so this patch tries to add a --nocow option to vol-create
>     functions and
>     > vol-clone function, so that users could have a chance to set
>     NOCOW to a new
>     > volume if that happens to create on a btrfs like file system.
>
>     What effect / impact does setting this flag have from a functional
>     POV ? 
>
>  
> It implies nodatasum as well. But COW may still happen if a snapshot
> is taken.
>
> Following is quoted from:
>  https://btrfs.wiki.kernel.org/index.php/FAQ
>  
>  Can copy-on-write be turned off for data blocks?
>
>  Yes, there are several ways how to do that.
>
>  Disable it by mounting with nodatacow. This implies nodatasum as
> well. COW may
> still happen if a snapshot is taken. However COW will still be
> maintained for
> existing files, because the COW status can be modified only for empty
> or newly
> created files.
>
>  For an empty file, add the NOCOW file attribute (use chattr utility
> with +C),
>  or you create a new file in a directory with the NOCOW attribute set
> (then the
>  new file will inherit this attribute). Now copy the original data
> into the
>  pre-created file, delete original and rename back.
>
>      Why would we not just unconditonally enable it on btrfs so
>     it was fast "out of the box" ?
>
>
>  COW is default feature of Btrfs. There are many advantages with COW
> mechanism.
>  Other uses may want the COW advantages at the same time we set NOCOW
> to a VM
>  image.
>
>  But in pool-create and vol-create case, it seems the whole pool is used
>  to hold VM images, so maybe we could just disable COW in pool side.
> Then all
>  vol created in it will be NOCOW. That means, in pool-start phase, if
> checking
>  fs format is 'btrfs', add '-o nodatacow' option to 'mount' command.
> That still need some
>  change in libvirt code. How do you think about this way?

Daniel,

Any thoughts on Chunyan's suggestion?  It seems mounting a btrfs pool
with nodatacow provides out-of-the-box performance improvement, but
doesn't later preclude creating/using COW volumes from the pool.

Regards,
Jim