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Date: Mon, 24 Jan 2011 15:44:16 -0200
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [Qemu-devel] [PATCH 1/3] block: add block_resize monitor command
Message-ID: <20110124174416.GA6178@amt.cnet>
References: <20110124123223.GA16558@lst.de>
 <20110124123233.GA16613@lst.de>
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To: Christoph Hellwig <hch@lst.de>
Cc: qemu-devel@nongnu.org

On Mon, Jan 24, 2011 at 01:32:33PM +0100, Christoph Hellwig wrote:
> Add a monitor command that allows resizing of block devices while
> qemu is running.  It uses the existing bdrv_truncate method already
> used by qemu-img to do it's work.  Compared to qemu-img the size
> parsing is very simplicistic, but I think having a properly numering
> object is more useful for non-humand monitor users than having
> the units and relative resize parsing.
> 
> For SCSI devices the new size can be updated in Linux guests by
> doing the following shell command:
> 
> 	echo > /sys/class/scsi_device/0:0:0:0/device/rescan
> 
> For ATA devices I don't know of a way to update the block device
> size in Linux system, and for virtio-blk the next two patches
> will provide an automatic update of the size when this command
> is issued on the host.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Index: qemu/hmp-commands.hx
> ===================================================================
> --- qemu.orig/hmp-commands.hx	2011-01-24 11:55:36.744254374 +0100
> +++ qemu/hmp-commands.hx	2011-01-24 11:56:23.619254094 +0100
> @@ -53,6 +53,25 @@ Quit the emulator.
>  ETEXI
>  
>      {
> +        .name       = "block_resize",
> +        .args_type  = "device:B,size:o",
> +        .params     = "device size",
> +        .help       = "resize a block image",
> +        .user_print = monitor_user_noop,
> +        .mhandler.cmd_new = do_block_resize,
> +    },
> +
> +STEXI
> +@item block_resize
> +@findex block_resize
> +Resize a block image while a guest is running.  Usually requires guest
> +action to see the updated size.  Resize to a lower size is supported,
> +but should be used with extreme caution.  Note that this command only
> +resizes image files, it can not resize block devices like LVM volumes.
> +ETEXI
> +
> +
> +    {
>          .name       = "eject",
>          .args_type  = "force:-f,device:B",
>          .params     = "[-f] device",
> Index: qemu/blockdev.c
> ===================================================================
> --- qemu.orig/blockdev.c	2011-01-24 11:56:20.903004129 +0100
> +++ qemu/blockdev.c	2011-01-24 11:56:38.391254165 +0100
> @@ -705,3 +705,33 @@ int do_drive_del(Monitor *mon, const QDi
>  
>      return 0;
>  }
> +
> +/*
> + * XXX: replace the QERR_UNDEFINED_ERROR errors with real values once the
> + * existing QERR_ macro mess is cleaned up.  A good example for better
> + * error reports can be found in the qemu-img resize code.
> + */
> +int do_block_resize(Monitor *mon, const QDict *qdict, QObject **ret_data)
> +{
> +    const char *device = qdict_get_str(qdict, "device");
> +    int64_t size = qdict_get_int(qdict, "size");
> +    BlockDriverState *bs;
> +
> +    bs = bdrv_find(device);
> +    if (!bs) {
> +        qerror_report(QERR_DEVICE_NOT_FOUND, device);
> +        return -1;
> +    }
> +
> +    if (size < 0) {
> +        qerror_report(QERR_UNDEFINED_ERROR);
> +        return -1;
> +    }
> +
> +    if (bdrv_truncate(bs, size)) {
> +        qerror_report(QERR_UNDEFINED_ERROR);
> +        return -1;
> +    }

Can't resize if block migration is in progress. Don't see a problem 
with simply disallowing resize in that case.