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Date: Tue, 06 Dec 2011 16:20:55 +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 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.

Kevin