diff options
| author | aliguori <aliguori> | 2008-12-04 21:39:21 +0000 |
|---|---|---|
| committer | aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162> | 2008-12-04 21:39:21 +0000 |
| commit | e724781ef42d11c7de95b3df58fa61bbb3b1bf80 (patch) | |
| tree | 6bc9f76c577689689ea9b780994cbe544a4e8a71 /qemu/qemu-doc.texi | |
| parent | 17ef75d595aad352308f1dcd4a01117a483d8c4a (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>
Diffstat (limited to 'qemu/qemu-doc.texi')
| -rw-r--r-- | qemu/qemu-doc.texi | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/qemu/qemu-doc.texi b/qemu/qemu-doc.texi index f2c56ce0..77170d3a 100644 --- a/qemu/qemu-doc.texi +++ b/qemu/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 |