tune2fs -E hash_alg=half_md4: safe for existing filesystems?

[Bill Davidsen]

Bryn M. Reeves wrote:
> On 01/09/2013 06:18 PM, Bill Davidsen wrote:
>> I would try this on something you can afford to lose... I just don't see
>> any more info on this than you did before asking. I am curious, since I
>
> Changing hash_alg on an existing file system sets the default hash algorithm for
> newly created directories. It won't affect any existing directory inodes that
> are already present (even if you remove all the files within them - you need to
> remove and re-create the directory itself to switch algorithm).
>
> File systems with mixed hash algorithms probably see a lot less testing than
> either one or the other but both options have been around for quite a while now.
>
Thank you for the information below, I can certainly do a test and see if it 
makes a big difference, some testing will be needed.

> You can test this easily enough on loopback devices if needed and there's no
> real storage available and use debugfs to inspect the hash used for a particular
> directory, e.g.:
>
> debugfs> htree foo
> Root node dump:
>       Reserved zero: 0
>       Hash Version: 2
>       Info length: 8
>       Indirect levels: 0
>       Flags: 0
> Number of entries (count): 2
> Number of entries (limit): 124
> Entry #0: Hash 0x00000000, block 1
> Entry #1: Hash 0x896bc308, block 2
> [...]
>
> debugfs> htree bar
> Root node dump:
>       Reserved zero: 0
>       Hash Version: 1
>       Info length: 8
>       Indirect levels: 0
>       Flags: 0
> Number of entries (count): 2
> Number of entries (limit): 124
> Entry #0: Hash 0x00000000, block 1
> Entry #1: Hash 0x7aa4fee4, block 2
> [...]
>
> The hash versions are as follows:
>
> #define EXT2_HASH_LEGACY                0
> #define EXT2_HASH_HALF_MD4              1
> #define EXT2_HASH_TEA                   2
>
> It's probably somewhat more risky than re-making the file system with the
> preferred hash and restoring but you could progressively migrate a file system
> over (or at least do some testing to see if it's worth it or not) by copying and
> renaming existing trees after changing hash_alg.
>
> Regards,
> Bryn.
>


-- 
Bill Davidsen <davidsen at tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot