linux-ext4 - Regarding ext4 extent allocation strategy

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Date:   Tue, 13 Jul 2021 12:22:14 +0530
From:   Shyam Prasad N <nspmangalore@...il.com>
To:     tytso@....edu, David Howells <dhowells@...hat.com>,
        Steve French <smfrench@...il.com>, linux-ext4@...r.kernel.org
Subject: Regarding ext4 extent allocation strategy

Hi,

Our team in Microsoft, which works on the Linux SMB3 client kernel
filesystem has recently been exploring the use of fscache on top of
ext4 for caching the network filesystem data for some customer
workloads.

However, the maintainer of fscache (David Howells) recently warned us
that a few other extent based filesystem developers pointed out a
theoretical bug in the current implementation of fscache/cachefiles.
It currently does not maintain a separate metadata for the cached data
it holds, but instead uses the sparseness of the underlying filesystem
to track the ranges of the data that is being cached.
The bug that has been pointed out with this is that the underlying
filesystems could bridge holes between data ranges with zeroes or
punch hole in data ranges that contain zeroes. (@David please add if I
missed something).

David has already begun working on the fix to this by maintaining the
metadata of the cached ranges in fscache itself.
However, since it could take some time for this fix to be approved and
then backported by various distros, I'd like to understand if there is
a potential problem in using fscache on top of ext4 without the fix.
If ext4 doesn't do any such optimizations on the data ranges, or has a
way to disable such optimizations, I think we'll be okay to use the
older versions of fscache even without the fix mentioned above.

Opinions?

-- 
Regards,
Shyam