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Re: Writing journal only in Big Endian format

To: Nitin Arora <nitin.arora.del@xxxxxxxxx>
Subject: Re: Writing journal only in Big Endian format
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 30 Nov 2009 17:43:56 +1100
Cc: xfs@xxxxxxxxxxx, "sandeen-xfs@xxxxxxxxxxx" <sandeen-xfs@xxxxxxxxxxx>, "p.mironchik@xxxxxxxxxxx" <p.mironchik@xxxxxxxxxxx>, "hch@xxxxxxx" <hch@xxxxxxx>
In-reply-to: <409bd82b0911292117n5aa11961u86e787d20acbd2b3@xxxxxxxxxxxxxx>
References: <409bd82b0911292117n5aa11961u86e787d20acbd2b3@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
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
method....

> 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.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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