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

2013/12/5 Daniel P. Berrange <berrange(a)redhat.com&gt; 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 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? Thanks, Chunyan