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-rw-r--r--qemu-doc.texi6
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diff --git a/qemu-doc.texi b/qemu-doc.texi
index f2c56ce..77170d3 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
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