Re: [libvirt] [PATCH V2] storagevol: add nocow to vol xml

Add 'nocow' to storage volume xml so that user can have an option to set NOCOW flag to the newly created volume. It's useful on btrfs file system to enhance performance. Btrfs has low 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. Generally, there are two ways to turn off NOCOW on btrfs: a) by mounting fs with nodatacow,

then all newly created files will be NOCOW. b) per file. Add the NOCOW file attribute. It could only be done to empty or new files. This patch tries the second way, according to 'nocow' option, it could set NOCOW flag per file: for raw file images, handle 'nocow' in libvirt code; for non-raw file images, pass 'nocow=on' option to qemu-img, and let qemu-img to handle that (requires qemu-img version >= 2.1). Signed-off-by: Chunyan Liu <cyliu(a)suse.com&gt; --- Changes: - now qemu-img can handle 'nocow=on' option, just pass 'nocow=on' to qemu-img for non-raw file images. No need to handle all file type in libvirt code. Pervious version is here: http://www.redhat.com/archives/libvir-list/2013-December/msg01257.html --- docs/formatstorage.html.in | 7 +++++++ docs/schemas/storagevol.rng | 5 +++++

src/conf/storage_conf.c | 3 +++ src/storage/storage_backend.c | 22 ++++++++++++++++++++++ src/util/virstoragefile.h | 1 + 5 files changed, 38 insertions(+) diff --git a/docs/formatstorage.html.in b/docs/formatstorage.html.in index 1cd82b4..e8862bf 100644 --- a/docs/formatstorage.html.in +++ b/docs/formatstorage.html.in @@ -385,6 +385,7 @@ &lt;label&gt;virt_image_t&lt;/label&gt; &lt;/permissions&gt; &lt;compat&gt;1.1&lt;/compat&gt; + &lt;nocow/&gt; &lt;features&gt; &lt;lazy_refcounts/&gt; &lt;/features&gt; @@ -424,6 +425,12 @@ 1.1 is used. If omitted, qemu-img default is used. <span class="since">Since 1.1.0</span> </dd> + <dt><code>nocow</code></dt> + <dd>Turn off COW of the newly created volume. So far, this is only valid + to a file image in btrfs file system. It will improve performance when

+ the file image is used in VM. To create non-raw file images, it + requires QEMU version since 2.1. <span class="since">Since 1.2.6</span>