aboutsummaryrefslogtreecommitdiff
path: root/qemu-doc.texi
diff options
context:
space:
mode:
authoraliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>2008-12-04 21:39:21 +0000
committeraliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>2008-12-04 21:39:21 +0000
commit4dc822d726376fd4369089f04eb8605d2f94b74f (patch)
tree743ea1d2ed14426c2d9f3da8d48d24b14f990df0 /qemu-doc.texi
parent0cd2df75a2d7223b567c0eaa2547ce6c7d6a83f7 (diff)
Use writeback caching by default with qcow2
qcow2 writes a cluster reference count on every cluster update. This causes performance to crater when using anything but cache=writeback. This is most noticeable when using savevm. Right now, qcow2 isn't a reliable format regardless of the type of cache your using because metadata is not updated in the correct order. Considering this, I think it's somewhat reasonable to use writeback caching by default with qcow2 files. It at least avoids the massive performance regression for users until we sort out the issues in qcow2. Signed-off-by: Anthony Liguori <aliguori@us.ibm.com> git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5879 c046a42c-6fe2-441c-8c8c-71466251a162
Diffstat (limited to 'qemu-doc.texi')
-rw-r--r--qemu-doc.texi6
1 files changed, 6 insertions, 0 deletions
diff --git a/qemu-doc.texi b/qemu-doc.texi
index f2c56ce0e0..77170d3ad6 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -289,6 +289,12 @@ The host page can be avoided entirely with @option{cache=none}. This will
attempt to do disk IO directly to the guests memory. QEMU may still perform
an internal copy of the data.
+Some block drivers perform badly with @option{cache=writethrough}, most notably,
+qcow2. If performance is more important than correctness,
+@option{cache=writeback} should be used with qcow2. By default, if no explicit
+caching is specified for a qcow2 disk image, @option{cache=writeback} will be
+used. For all other disk types, @option{cache=writethrough} is the default.
+
Instead of @option{-cdrom} you can use:
@example
qemu -drive file=file,index=2,media=cdrom