From: Prashant Shah <pshah.mumbai@gmail.com>
Subject: Re: Fwd: block level cow operation
Date: Thu, 25 Apr 2013 18:30:18 +0530
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 <CAD6i1f+NLF6Vj8D84FFWAbDttqbBvcg-kWswf7Hez2o0-cXpMw@mail.gmail.com> <87txnfrcbp.fsf@openvz.org>
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Cc: linux-ext4@vger.kernel.org
To: Dmitry Monakhov <dmonakhov@openvz.org>
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Hi,

On Tue, Apr 9, 2013 at 8:16 PM, Dmitry Monakhov <dmonakhov@openvz.org> wrote:
>
> you should not block bio/requests handling, but simply deffer original
> bio. Some things like that:
>
> OUR_MAIN_ENTERING_POINT {
>   if (bio->bi_rw == WRITE) {
>      if (cow_required(bio))
>        cow_bio  = create_cow_copy(bio)
>        submit_bio(cow_bio);
>    }
>   /* Cow is not required */
>    submit_bio(bio);
> }

> This approach gives us reasonable performance ~3 times slower than disk
> throughput.
> For a reference implementation you may look at driver/dm/dm-snap or to
> Acronis snapapi module (AFAIR it is opensource)
> }

Thanks. That is what I was looking for. Got the ref code from snapapi
module which is opensource.

Its not something that is specific to any filesystem.

Regards.