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Re: [Qemu-devel] [PATCH v5] block:add-cow file format

From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v5] block:add-cow file format
Date: Tue, 06 Dec 2011 16:20:55 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

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.


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