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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 16:11:07 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0

Am 06.12.2011 15:53, schrieb Marcelo Tosatti:
> 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.
> 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?

The obvious case is raw-win32. There are probably not so obvious, but
still valid use cases that involve things like NBD, iSCSI, blkdebug or


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