virtio-9p: Rearrange fileop structures

[mirror_qemu.git] / qemu-options.hx

diff --git a/qemu-options.hx b/qemu-options.hx
index 03e95fd2bf24c8ea5dfd2152c4c132d0ff805496..a6928b7aa3bbe759465d6dde2998fb41c1f4eade 100644 (file)
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -118,8 +118,9 @@ ETEXI
 DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "-drive [file=file][,if=type][,bus=n][,unit=m][,media=d][,index=i]\n"
     "       [,cyls=c,heads=h,secs=s[,trans=t]][,snapshot=on|off]\n"
-    "       [,cache=writethrough|writeback|none][,format=f][,serial=s]\n"
-    "       [,addr=A][,id=name][,aio=threads|native][,readonly=on|off]\n"
+    "       [,cache=writethrough|writeback|unsafe|none][,format=f]\n"
+    "       [,serial=s][,addr=A][,id=name][,aio=threads|native]\n"
+    "       [,readonly=on|off]\n"
     "                use 'file' as a drive image\n", QEMU_ARCH_ALL)
 STEXI
 @item -drive @var{option}[,@var{option}[,@var{option}[,...]]]
@@ -148,7 +149,7 @@ 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 "none", "writeback", or "writethrough" and controls how the host cache is used to access block data.
+@var{cache} is "none", "writeback", "unsafe", or "writethrough" and controls how the host cache is used to access block data.
 @item aio=@var{aio}
 @var{aio} is "threads", or "native" and selects between pthread based disk I/O and native Linux AIO.
 @item format=@var{format}
@@ -169,8 +170,7 @@ 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.
+corruption.
 
 The host page cache can be avoided entirely with @option{cache=none}.  This will
 attempt to do disk IO directly to the guests memory.  QEMU may still perform
@@ -180,6 +180,13 @@ 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.
 
+In case you don't care about data integrity over host failures, use
+cache=unsafe. This option tells qemu that it never needs to write any data
+to the disk but can instead keeps things in cache. If anything goes wrong,
+like your host losing power, the disk storage getting disconnected accidently,
+etc. you're image will most probably be rendered unusable.   When using
+the @option{-snapshot} option, unsafe caching is always used.
+
 Instead of @option{-cdrom} you can use:
 @example
 qemu -drive file=file,index=2,media=cdrom