path: root/qemu-img.texi
diff options
authorChunyan Liu <cyliu@suse.com>2014-06-30 14:29:58 +0800
committerStefan Hajnoczi <stefanha@redhat.com>2014-07-01 10:15:12 +0200
commit4ab1559085d688dddf4de77f1ead3102e243e668 (patch)
treec02aaa834a94aace2ef773462cea3b4829cc48a9 /qemu-img.texi
parent8698e110f8aa1cdf8ba1afdda8f2658333a4ab01 (diff)
qemu-img create: add 'nocow' option
Add 'nocow' option so that users could have a chance to set NOCOW flag to newly created files. 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 the option, it could add NOCOW per file. For most block drivers, since the create file step is in raw-posix.c, so we can do setting NOCOW flag ioctl in raw-posix.c only. But there are some exceptions, like block/vpc.c and block/vdi.c, they are creating file by calling qemu_open directly. For them, do the same setting NOCOW flag ioctl work in them separately. [Fixed up 082.out due to the new 'nocow' creation option --Stefan] Signed-off-by: Chunyan Liu <cyliu@suse.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'qemu-img.texi')
1 files changed, 16 insertions, 0 deletions
diff --git a/qemu-img.texi b/qemu-img.texi
index c68b54148a..8496f3b8dc 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -474,6 +474,22 @@ check -r all} is required, which may take some time.
This option can only be enabled if @code{compat=1.1} is specified.
+@item nocow
+If this option is set to @code{on}, it will trun off COW of the file. It's only
+valid on btrfs, no effect on other file systems.
+Btrfs has low performance when hosting a VM image file, even more when the guest
+on the VM also using btrfs as file system. Turning off COW is a way to mitigate
+this bad performance. Generally there are two ways to turn off COW on btrfs:
+a) Disable it by mounting with nodatacow, then all newly created files will be
+NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
+Note: this option is only valid to new or empty files. If there is an existing
+file which is COW and has data blocks already, it couldn't be changed to NOCOW
+by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
+the NOCOW flag is set or not (Capitabl 'C' is NOCOW flag).
@end table
@item Other