From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from [140.186.70.92] (port=44612 helo=eggs.gnu.org)
	by lists.gnu.org with esmtp (Exim 4.43) id 1PznTO-0001s4-Kn
	for qemu-devel@nongnu.org; Wed, 16 Mar 2011 05:56:59 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <kwolf@redhat.com>) id 1PznTM-00028j-Nu
	for qemu-devel@nongnu.org; Wed, 16 Mar 2011 05:56:57 -0400
Received: from mx1.redhat.com ([209.132.183.28]:37249)
	by eggs.gnu.org with esmtp (Exim 4.71)
	(envelope-from <kwolf@redhat.com>) id 1PznTM-00028b-7A
	for qemu-devel@nongnu.org; Wed, 16 Mar 2011 05:56:56 -0400
Message-ID: <4D8089E5.3060606@redhat.com>
Date: Wed, 16 Mar 2011 10:59:01 +0100
From: Kevin Wolf <kwolf@redhat.com>
MIME-Version: 1.0
References: <20110315141049.GA30627@lst.de>	<20110315141132.GB30710@lst.de>
	<AANLkTikZaDcgBs3RET+S8YRZhuCupp7_0CYuKqpmt+MY@mail.gmail.com>
In-Reply-To: <AANLkTikZaDcgBs3RET+S8YRZhuCupp7_0CYuKqpmt+MY@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: [Qemu-devel] Re: [PATCH 2/4] block: add a helper to change
 writeback mode on the fly
List-Id: qemu-devel.nongnu.org
List-Unsubscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <http://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>, aliguori@us.ibm.com, prerna@linux.vnet.ibm.com, Christoph Hellwig <hch@lst.de>, qemu-devel@nongnu.org

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