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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 17:43:45 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

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.


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