linux-ext4 - Re: 16 TB filesystem limit on 32bit machine

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Message-ID: <4F42CE49.80209@redhat.com>
Date:	Mon, 20 Feb 2012 16:50:49 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Phillip Susi <psusi@...ntu.com>
CC:	Rabeeh Khoury <rabeeh3000@...il.com>, linux-ext4@...r.kernel.org
Subject: Re: 16 TB filesystem limit on 32bit machine

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On 2/20/12 3:17 PM, Phillip Susi wrote:
> On 2/20/2012 11:50 AM, Eric Sandeen wrote:
>> fs metadata is mapped into an address space, IIRC, so can't be
>> addressed past 2^32 pages.  Also, mkfs can't do buffered IO to the
>> device past 16T (it is writing to a device _file_) and ditto for
>> e2fsck.
> 
> But the file is only used for open(), after that the IO is handled by
> the correct device driver, which handles 64 bit offsets ( when you
> have CONFIG_LBDAF on ).  So if you can change the page cache index
> size ( and it doesn't blow up ) this should work fine.

Oh, sure - if you change pgoff_t to 64 bits, but until then, you
can't even mkfs.ext4 a device larger than 16T; there is nowhere
for that buffered IO to go in the page cache, right?

(You could mkfs.ext4 a 16T partition on a 40T lun today, thanks
to LBDAF, but not a 17T without changing pgoff_t et al)

And "if you change the page cache index size" that's a big leap
of faith, today.  Might be interesting work to do, but I'd bet
it's a real chunk of work.

- -Eric
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