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


From: Marcelo Tosatti
Subject: Re: [Qemu-devel] [PATCH v5] block:add-cow file format
Date: Tue, 6 Dec 2011 12:53:16 -0200
User-agent: Mutt/1.5.21 (2010-09-15)

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?




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