Re: [RFC 0/13] extents and 48bit ext3

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On Jun 9, 2006, at 11:25:31, Jeff Garzik wrote:
Overall, I'm surprised that ext3 developers don't see any of the problems related to progressive, stealth filesystem upgrades.

Users are never given a clear indication of when their metadata is being upgraded, there is no clear "line of demarcation" they cross, when they start using extents.

Since there is no user-visible fs upgrade event, users do not have a clear picture of what features are being used -- which means they are kept in the dark about which kernels are OK to use on their data.

Do you guys honestly expect users to keep track of which kernels added specific ext3 features?

This is why other enterprise filesystems have clear "fs version 1", "fs version 2" points across which a user migrates. ext3's feature- flags approach just means that there are a million combinations of potential old-and-new features, in-tree and third party, all of which must be supported.

One possible solution to the version-confusion that would avoid duplicating features would be to merge the fs/ext{2,3} to fs/ext, then make fs/ext register itself as a filesystem under "ext2", "ext3", and "ext4". Then have each name imply a specific set of features and compatibility. That would allow the same performance optimizations to affect all 3 even as you make metadata changes in the latest version. I've heard quite some griping about the amount of duplicated code between ext2 and ext3; why cause those problems again with an "ext4"? There would probably be some fs/ext/ext{2,3,4} _foo.c files that could be compiled in or out depending on configured FS support, but I would guess that would make it easier on users and developers alike.

Cheers,
Kyle Moffett

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