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Diffstat (limited to 'qemu-img.texi')
| -rw-r--r-- | qemu-img.texi | 16 |
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 +does. + +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 |