doc: Document generic -blockdev options
This adds documentation for the -blockdev options that apply to all nodes independent of the block driver used. All options that are shared by -blockdev and -drive are now explained in the section for -blockdev. The documentation of -drive mentions that all -blockdev options are accepted as well. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Kevin Wolf
parent
8649f2f9b2
commit
dfaca4641c
108
qemu-options.hx
108
qemu-options.hx
@ -610,6 +610,53 @@ DEF("blockdev", HAS_ARG, QEMU_OPTION_blockdev,
|
||||
" [,read-only=on|off][,detect-zeroes=on|off|unmap]\n"
|
||||
" [,driver specific parameters...]\n"
|
||||
" configure a block backend\n", QEMU_ARCH_ALL)
|
||||
STEXI
|
||||
@item -blockdev @var{option}[,@var{option}[,@var{option}[,...]]]
|
||||
@findex -blockdev
|
||||
|
||||
Define a new block driver node.
|
||||
|
||||
@table @option
|
||||
@item Valid options for any block driver node:
|
||||
|
||||
@table @code
|
||||
@item driver
|
||||
Specifies the block driver to use for the given node.
|
||||
@item node-name
|
||||
This defines the name of the block driver node by which it will be referenced
|
||||
later. The name must be unique, i.e. it must not match the name of a different
|
||||
block driver node, or (if you use @option{-drive} as well) the ID of a drive.
|
||||
|
||||
If no node name is specified, it is automatically generated. The generated node
|
||||
name is not intended to be predictable and changes between QEMU invocations.
|
||||
For the top level, an explicit node name must be specified.
|
||||
@item read-only
|
||||
Open the node read-only. Guest write attempts will fail.
|
||||
@item cache.direct
|
||||
The host page cache can be avoided with @option{cache.direct=on}. This will
|
||||
attempt to do disk IO directly to the guest's memory. QEMU may still perform an
|
||||
internal copy of the data.
|
||||
@item cache.no-flush
|
||||
In case you don't care about data integrity over host failures, you can use
|
||||
@option{cache.no-flush=on}. This option tells QEMU that it never needs to write
|
||||
any data to the disk but can instead keep things in cache. If anything goes
|
||||
wrong, like your host losing power, the disk storage getting disconnected
|
||||
accidentally, etc. your image will most probably be rendered unusable.
|
||||
@item discard=@var{discard}
|
||||
@var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and controls
|
||||
whether @code{discard} (also known as @code{trim} or @code{unmap}) requests are
|
||||
ignored or passed to the filesystem. Some machine types may not support
|
||||
discard requests.
|
||||
@item detect-zeroes=@var{detect-zeroes}
|
||||
@var{detect-zeroes} is "off", "on" or "unmap" and enables the automatic
|
||||
conversion of plain zero writes by the OS to driver specific optimized
|
||||
zero write commands. You may even choose "unmap" if @var{discard} is set
|
||||
to "unmap" to allow a zero write to be converted to an @code{unmap} operation.
|
||||
@end table
|
||||
|
||||
@end table
|
||||
|
||||
ETEXI
|
||||
|
||||
DEF("drive", HAS_ARG, QEMU_OPTION_drive,
|
||||
"-drive [file=file][,if=type][,bus=n][,unit=m][,media=d][,index=i]\n"
|
||||
@ -630,7 +677,12 @@ STEXI
|
||||
@item -drive @var{option}[,@var{option}[,@var{option}[,...]]]
|
||||
@findex -drive
|
||||
|
||||
Define a new drive. Valid options are:
|
||||
Define a new drive. This includes creating a block driver node (the backend) as
|
||||
well as a guest device, and is mostly a shortcut for defining the corresponding
|
||||
@option{-blockdev} and @option{-device} options.
|
||||
|
||||
@option{-drive} accepts all options that are accepted by @option{-blockdev}. In
|
||||
addition, it knows the following options:
|
||||
|
||||
@table @option
|
||||
@item file=@var{file}
|
||||
@ -657,11 +709,31 @@ These options have the same definition as they have in @option{-hdachs}.
|
||||
@var{snapshot} is "on" or "off" and controls snapshot mode for the given drive
|
||||
(see @option{-snapshot}).
|
||||
@item cache=@var{cache}
|
||||
@var{cache} is "none", "writeback", "unsafe", "directsync" or "writethrough" and controls how the host cache is used to access block data.
|
||||
@var{cache} is "none", "writeback", "unsafe", "directsync" or "writethrough"
|
||||
and controls how the host cache is used to access block data. This is a
|
||||
shortcut that sets the @option{cache.direct} and @option{cache.no-flush}
|
||||
options (as in @option{-blockdev}), and additionally @option{cache.writeback},
|
||||
which provides a default for the @option{write-cache} option of block guest
|
||||
devices (as in @option{-device}). The modes correspond to the following
|
||||
settings:
|
||||
|
||||
@c Our texi2pod.pl script doesn't support @multitable, so fall back to using
|
||||
@c plain ASCII art (well, UTF-8 art really). This looks okay both in the manpage
|
||||
@c and the HTML output.
|
||||
@example
|
||||
@ │ cache.writeback cache.direct cache.no-flush
|
||||
─────────────┼─────────────────────────────────────────────────
|
||||
writeback │ on off off
|
||||
none │ on on off
|
||||
writethrough │ off off off
|
||||
directsync │ off on off
|
||||
unsafe │ on off on
|
||||
@end example
|
||||
|
||||
The default mode is @option{cache=writeback}.
|
||||
|
||||
@item aio=@var{aio}
|
||||
@var{aio} is "threads", or "native" and selects between pthread based disk I/O and native Linux AIO.
|
||||
@item discard=@var{discard}
|
||||
@var{discard} is one of "ignore" (or "off") or "unmap" (or "on") and controls whether @dfn{discard} (also known as @dfn{trim} or @dfn{unmap}) requests are ignored or passed to the filesystem. Some machine types may not support discard requests.
|
||||
@item format=@var{format}
|
||||
Specify which disk @var{format} will be used rather than detecting
|
||||
the format. Can be used to specify format=raw to avoid interpreting
|
||||
@ -676,16 +748,9 @@ Specify which @var{action} to take on write and read errors. Valid actions are:
|
||||
"report" (report the error to the guest), "enospc" (pause QEMU only if the
|
||||
host disk is full; report the error to the guest otherwise).
|
||||
The default setting is @option{werror=enospc} and @option{rerror=report}.
|
||||
@item readonly
|
||||
Open drive @option{file} as read-only. Guest write attempts will fail.
|
||||
@item copy-on-read=@var{copy-on-read}
|
||||
@var{copy-on-read} is "on" or "off" and enables whether to copy read backing
|
||||
file sectors into the image file.
|
||||
@item detect-zeroes=@var{detect-zeroes}
|
||||
@var{detect-zeroes} is "off", "on" or "unmap" and enables the automatic
|
||||
conversion of plain zero writes by the OS to driver specific optimized
|
||||
zero write commands. You may even choose "unmap" if @var{discard} is set
|
||||
to "unmap" to allow a zero write to be converted to an UNMAP operation.
|
||||
@item bps=@var{b},bps_rd=@var{r},bps_wr=@var{w}
|
||||
Specify bandwidth throttling limits in bytes per second, either for all request
|
||||
types or for reads or writes only. Small values can lead to timeouts or hangs
|
||||
@ -712,34 +777,19 @@ prevent guests from circumventing throttling limits by using many small disks
|
||||
instead of a single larger disk.
|
||||
@end table
|
||||
|
||||
By default, the @option{cache=writeback} mode is used. It will report data
|
||||
By default, the @option{cache.writeback=on} mode is used. It will report data
|
||||
writes as completed as soon as the data is present in the host page cache.
|
||||
This is safe as long as your guest OS makes sure to correctly flush disk caches
|
||||
where needed. If your guest OS does not handle volatile disk write caches
|
||||
correctly and your host crashes or loses power, then the guest may experience
|
||||
data corruption.
|
||||
|
||||
For such guests, you should consider using @option{cache=writethrough}. This
|
||||
For such guests, you should consider using @option{cache.writeback=off}. 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 after QEMU has made sure to flush
|
||||
each write to the disk. Be aware that this has a major impact on performance.
|
||||
|
||||
The host page cache can be avoided entirely with @option{cache=none}. This will
|
||||
attempt to do disk IO directly to the guest's memory. QEMU may still perform
|
||||
an internal copy of the data. Note that this is considered a writeback mode and
|
||||
the guest OS must handle the disk write cache correctly in order to avoid data
|
||||
corruption on host crashes.
|
||||
|
||||
The host page cache can be avoided while only sending write notifications to
|
||||
the guest when the data has been flushed to the disk using
|
||||
@option{cache=directsync}.
|
||||
|
||||
In case you don't care about data integrity over host failures, use
|
||||
@option{cache=unsafe}. This option tells QEMU that it never needs to write any
|
||||
data to the disk but can instead keep things in cache. If anything goes wrong,
|
||||
like your host losing power, the disk storage getting disconnected accidentally,
|
||||
etc. your image will most probably be rendered unusable. When using
|
||||
the @option{-snapshot} option, unsafe caching is always used.
|
||||
When using the @option{-snapshot} option, unsafe caching is always used.
|
||||
|
||||
Copy-on-read avoids accessing the same backing file sectors repeatedly and is
|
||||
useful when the backing file is over a slow network. By default copy-on-read
|
||||
|
||||
Loading…
Reference in New Issue
Block a user