Re: [Qemu-devel] [PATCHv3 3/6] block: add a knob to disable multiwrite_merge

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sat, Oct 25, 2014 at 06:55:50PM +0200, Peter Lieven wrote:
> diff --git a/block.c b/block.c
> index f05ea0c..f3da5dd 100644
> --- a/block.c
> +++ b/block.c
> @@ -884,6 +884,10 @@ static QemuOptsList bdrv_runtime_opts = {
>              .name = "node-name",
>              .type = QEMU_OPT_STRING,
>              .help = "Node name of the block device node",
> +        },{
> +            .name = "write-merging",
> +            .type = QEMU_OPT_BOOL,
> +            .help = "enable write merging (default: true)",

QEMU_OPT_BOOL takes "on"/"off" instead of "true"/"false":
s/true/on/

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 2095f9e..74d1960 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -214,6 +214,8 @@
>  #
>  # @detect_zeroes: detect and optimize zero writes (Since 2.1)
>  #
> +# @write_merging: true if write merging is enabled (Since 2.2)
> +#
>  # @bps: total throughput limit in bytes per second is specified
>  #
>  # @bps_rd: read throughput limit in bytes per second is specified
> @@ -250,6 +252,7 @@
>              '*backing_file': 'str', 'backing_file_depth': 'int',
>              'encrypted': 'bool', 'encryption_key_missing': 'bool',
>              'detect_zeroes': 'BlockdevDetectZeroesOptions',
> +            'write_merging': 'bool',
>              'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
>              'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
>              'image': 'ImageInfo',
> @@ -1185,6 +1188,10 @@
>  #                 (default: false)
>  # @detect-zeroes: #optional detect and optimize zero writes (Since 2.1)
>  #                 (default: off)
> +# @write-merging: #optional enable the merging of write requests
> +#                 also known as multiwrite_merge (Since 2.2)
> +#                 (default: true, but this might change in the future
> +#                 depending on format/protocol/features used)
>  #
>  # Since: 1.7
>  ##
> @@ -1198,7 +1205,8 @@
>              '*rerror': 'BlockdevOnError',
>              '*werror': 'BlockdevOnError',
>              '*read-only': 'bool',
> -            '*detect-zeroes': 'BlockdevDetectZeroesOptions' } }
> +            '*detect-zeroes': 'BlockdevDetectZeroesOptions',
> +            '*write-merging': 'bool' } }
>  
>  ##
>  # @BlockdevOptionsFile
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 22cf3b9..d2f756f 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -432,6 +432,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
>      "       [,werror=ignore|stop|report|enospc][,id=name][,aio=threads|native]\n"
>      "       [,readonly=on|off][,copy-on-read=on|off]\n"
>      "       [,discard=ignore|unmap][,detect-zeroes=on|off|unmap]\n"
> +    "       [,write-merging=on|off]\n"
>      "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]]\n"
>      "       [[,iops=i]|[[,iops_rd=r][,iops_wr=w]]]\n"
>      "       [[,bps_max=bm]|[[,bps_rd_max=rm][,bps_wr_max=wm]]]\n"
> diff --git a/qmp-commands.hx b/qmp-commands.hx
> index 1abd619..2c20207 100644
> --- a/qmp-commands.hx
> +++ b/qmp-commands.hx
> @@ -2104,6 +2104,7 @@ Each json-object contain the following:
>           - "iops_size": I/O size when limiting by iops (json-int)
>           - "detect_zeroes": detect and optimize zero writing (json-string)
>               - Possible values: "off", "on", "unmap"
> +         - "write_merging": enable merging of write requests (json-bool)
>           - "image": the detail of the image, it is a json-object containing
>              the following:
>               - "filename": image file name (json-string)
> @@ -2181,6 +2182,7 @@ Example:
>                 "iops_wr_max": 0,
>                 "iops_size": 0,
>                 "detect_zeroes": "on",
> +               "write_merging": "true",
>                 "image":{
>                    "filename":"disks/test.qcow2",
>                    "format":"qcow2",

The big question is whether these user-visible interfaces make sense if
write merging will be moved from block.c into virtio-blk.c in the
future.  Once an interface has been added it cannot be removed.

I think write merging in block.c is fine since other emulated storage
controllers like virtio-scsi might also choose to use it.

But I want to raise the point for discussion in case others have strong
feelings.

Stefan

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