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Add cache=unsafe parameter to -drive

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Usually the guest can tell the host to flush data to disk. In some cases we
don't want to flush though, but try to keep everything in cache.

So let's add a new cache value to -drive that allows us to set the cache
policy to most aggressive, disabling flushes. We call this mode "unsafe",
as guest data is not guaranteed to survive host crashes anymore.

This patch also adds a noop function for aio, so we can do nothing in AIO
fashion.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
This commit is contained in:
Alexander Graf 2010-05-26 17:51:49 +02:00 • committed by Aurelien Jarno
parent d467b679f2
commit 016f5cf6ff
5 changed files with 44 additions and 4 deletions
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13
qemu-options.hx
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@ -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}
@ -180,6 +181,12 @@ 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.
Instead of @option{-cdrom} you can use:
@example
qemu -drive file=file,index=2,media=cdrom