linux-ext4 - bigalloc and max file size

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Date:	Wed, 26 Oct 2011 17:36:31 -0600
From:	Andreas Dilger <aedilger@...il.com>
To:	Theodore Tso <tytso@....edu>
Cc:	linux-ext4 development <linux-ext4@...r.kernel.org>,
	Alex Zhuravlev <bzzz@...mcloud.com>
Subject: bigalloc and max file size

Ted,
we were having a discussion about bigalloc and the maximum file size
(as limited by the 2^32 logical block number in struct ext4_extent).

Currently the maximum file size is blocksize * 2^32, 16TB for 4kB blocks.

Since it is not possible to allocate sub-blocks in the bigalloc code,
what about storing the "chunk number" in the extent logical block?

This would allow us to create files up to chunksize * 2^32.  With
a bigalloc chunk size of 1MB we could have a maximum file size of
2^(20 + 32) = 2^52 = 4PB, which is within spitting distance of the
maximum filesystem size of 2^56 bytes (4kB blocks * 2^48 blocks)
with the current extent format, and beyond reasonable limits today.

This essentially allows creating files as large as the filesystem size
without having to change the extent format, which is a good thing.

Is this implemented in bigalloc?  If not, it would be great to do
this before landing bigalloc in the upstream kernel, since it is
basically free to do, can already fall under the INCOMPAT_BIGALLOC
feature flag, and avoids issues in the near future.  I don't think
the e2fsprogs with bigalloc support is released yet either, so it
should still be OK to change the on-disk format?

Cheers, Andreas

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