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Date: Tue, 06 Dec 2011 17:43:45 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v5] block:add-cow file format
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To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: shuming@linux.vnet.ibm.com, Dong Xu Wang <wdongxu@linux.vnet.ibm.com>, qemu-devel@nongnu.org, stefanha@linux.vnet.ibm.com

Am 06.12.2011 17:35, schrieb Marcelo Tosatti:
> On Tue, Dec 06, 2011 at 04:20:55PM +0100, Kevin Wolf wrote:
>> Am 06.12.2011 16:06, schrieb Marcelo Tosatti:
>>> On Tue, Dec 06, 2011 at 12:53:16PM -0200, Marcelo Tosatti wrote:
>>>> On Tue, Dec 06, 2011 at 01:59:48PM +0100, Kevin Wolf wrote:
>>>>>>> +
>>>>>>> +    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
>>>>>>> +            state->bitmap_size);
>>>>>>> +    if (ret != state->bitmap_size) {
>>>>>>> +        goto fail;
>>>>>>> +    }
>>>>>>
>>>>>> Reading the entire bitmap in memory is not acceptable, it may be huge.
>>>>>> Better mmap it and use msync(MS_SYNC) when writing it back. This way the
>>>>>> host can free memory easily upon pressure.
>>>>>
>>>>> You can't use mmap in block drivers. It would only work with raw-posix
>>>>> backends, if at all.
>>>>>
>>>>> Kevin
>>>>
>>>> This is just the bitmap, a plain file. Why would you want to use
>>>> anything other than a plain file to use as storage for the bitmap?
>>>
>>> Well, mmap'ing would make life much simpler, but it has limitations such 
>>> as portability.
>>>
>>> Then what is necessary is a cache similar to qcow2's metadata cache.
>>
>> Right, we can probably generalise the qcow2 code and make it available
>> for other drivers as well.
> 
> Hum, generalising sounds overly complicated (and there is a time
> constraint to this). IMHO a cache internal to add-cow.c just to avoid
> reading the entire bitmap would do the trick.

The cache is mostly self-contained. But maybe we should get the locking
right (instead of always locking the whole BlockDriverState) before
using it in more drivers. I think this might need some change to it.

Kevin