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Diffstat (limited to 'qemu-doc.texi')
| -rw-r--r-- | qemu-doc.texi | 17 |
1 files changed, 16 insertions, 1 deletions
diff --git a/qemu-doc.texi b/qemu-doc.texi index adf270b470..84021fb816 100644 --- a/qemu-doc.texi +++ b/qemu-doc.texi @@ -267,13 +267,28 @@ These options have the same definition as they have in @option{-hdachs}. @item snapshot=@var{snapshot} @var{snapshot} is "on" or "off" and allows to enable snapshot for given drive (see @option{-snapshot}). @item cache=@var{cache} -@var{cache} is "on" or "off" and allows to disable host cache to access data. +@var{cache} is "none", "writeback", or "writethrough" and controls how the host cache is used to access block data. @item format=@var{format} Specify which disk @var{format} will be used rather than detecting the format. Can be used to specifiy format=raw to avoid interpreting an untrusted format header. @end table +By default, writethrough caching is used for all block device. This means that +the host page cache will be used to read and write data but write notification +will be sent to the guest only when the data has been reported as written by +the storage subsystem. + +Writeback caching will report data writes as completed as soon as the data is +present in the host page cache. This is safe as long as you trust your host. +If your host crashes or loses power, then the guest may experience data +corruption. When using the @option{-snapshot} option, writeback caching is +used by default. + +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. + Instead of @option{-cdrom} you can use: @example qemu -drive file=file,index=2,media=cdrom |