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Re: Writing journal only in Big Endian format

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: Writing journal only in Big Endian format
From: Nitin Arora <nitin.arora.del@xxxxxxxxx>
Date: Thu, 10 Dec 2009 13:56:32 +0530
Cc: xfs@xxxxxxxxxxx, "sandeen-xfs@xxxxxxxxxxx" <sandeen-xfs@xxxxxxxxxxx>, "p.mironchik@xxxxxxxxxxx" <p.mironchik@xxxxxxxxxxx>, "hch@xxxxxxx" <hch@xxxxxxx>
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Thanks Dave and Eric for your kind support.
We will then see if it is worth putting in effort for these changes or not.


On Mon, Nov 30, 2009 at 12:13 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
On Mon, Nov 30, 2009 at 10:47:41AM +0530, Nitin Arora wrote:
> The problem is that big endian machine cannot recognize the
> journal which was written in little endian format by the little
> endian machine and once the journal is zerod out it can be
> mouted.
> Now the solution to above problem seems writing journal always in
> big endian format.

Or you could just replay the log on the little endian machine and do
a clean unmount, then you should be able to move it to the big
endian machine without losing anything.

> Please suggest me, Is there any design limitation in XFS for this.

No design limitation, just a *lot* of work to change. If you don't
have a few months handy to work on this, I'd just use the above

> Is it okay and feasible to implement it if yes then please give
> some pointers so that it can be implemented,

All log headers, tails and sector fills need to be converted, every log
item format routine needs a big-endian version, log recovery needs
to be able to read both host-format and big-endian logs, all the
userspace tools need to learn about different log formats, QA tests
need to be written, etc.

> under some suitable
> compile time switch.

It could be done with a superblock feature bit, so the same kernel
could support both big-endian logs and host format.


Dave Chinner

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