[Qemu-devel] Re: [PATCH 2/4] block: add a helper to change writeback mode on the fly

[Kevin Wolf <kwolf@redhat.com>]

Am 16.03.2011 10:49, schrieb Stefan Hajnoczi:
> On Tue, Mar 15, 2011 at 2:11 PM, Christoph Hellwig <hch@lst.de> wrote:
>> Add a new bdrv_change_cache that can set/clear the writeback flag
>> at runtime by stopping all I/O and closing/reopening the image file.
>>
>> All code is based on a patch from Prerna Saxena <prerna@linux.vnet.ibm.com>
>> with minimal refactoring.
>>
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>>
>> Index: qemu/block.c
>> ===================================================================
>> --- qemu.orig/block.c   2011-03-15 11:47:31.285634626 +0100
>> +++ qemu/block.c        2011-03-15 14:57:03.680633093 +0100
>> @@ -441,6 +441,8 @@ static int bdrv_open_common(BlockDriverS
>>
>>     if (flags & BDRV_O_CACHE_WB)
>>         bs->enable_write_cache = 1;
>> +    else
>> +        bs->enable_write_cache = 0;
>>
>>     /*
>>      * Clear flags that are internal to the block layer before opening the
>> @@ -651,6 +653,44 @@ unlink_and_fail:
>>     return ret;
>>  }
>>
>> +static int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
>> +{
>> +    BlockDriver *drv = bs->drv;
>> +    int ret;
>> +
>> +    if (bdrv_flags == bs->open_flags) {
>> +        return 0;
>> +    }
>> +
>> +    /* Quiesce IO for the given block device */
>> +    qemu_aio_flush();
>> +    bdrv_flush(bs);
>> +
>> +    bdrv_close(bs);
>> +    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);
> 
> This will fail for -snapshot disks since the on-disk file is deleted.
> 
> In general it would be more robust to keep the original file
> descriptor around in case the file cannot be opened with the new
> flags.

Looks like we'll need a bdrv_reopen for protocols?

Kevin