Re: [rfc, patch 0/3] larger block group descriptors

Andreas Dilger wrote:
> On Jun 07, 2006  15:33 +0200, Alexandre Ratchov wrote:
>> as recently pointed out by Andreas, there isn't enough room in the block
>> group descriptor structure for all the fields we want to add in order to
>> extend the ext3 file system and/or to add new features.
>>
>> The attached patch addresses this problem, by allowing larger descriptor
>> sizes. The descriptor size is stored in the super-block, it must be a power
>> of 2, greater than or equal to 32 bytes (size of the current ext3_group_desc
>> structure) and must fit in one block.
>>
>> Temporary, i've added a new EXT3_FEATURE_INCOMPAT_DESC_SIZE feature. If
>> large descriptors are used by other patches we could just merge them and use
>> the same incompat feature.
> 
> Patches look good at first examination.  Use of INCOMPAT_DESC_SIZE should
> hopefully only be for testing, but possibly not a big deal either way as
> this patch will likely be tied to BIG_BG and other 64-bit changes.

I agree.

> I took a quick look bug haven't found a definitive "INCOMPAT_64BIT" patch
> in the current BULL/RH/IBM/CFS patchset.  There is INCOMPAT_EXTENTS, which
> is part of this, but there should likely be something else to ensure that
> 64-bit filesystem size in the superblock is flagged as an incompat change.
> 
> In the existing Bull patches RO_COMPAT_64BIT is used, but I don't think
> this is a safe read-only compat change, since an old kernel will just use
> s_blocks_count and not s_blocks_count_hi, truncating potentially a very
> large part of the filesystem.

I put RO_COMPAT to allow an old kernel to retrieve a part of data (when _hi part
is 0) if needed. But I have no definitive idea on this, if you think INCOMPAT_
is better I have no objection to use it.

Regards,
Laurent

-- 
Laurent Vivier
Bull, Architect of an Open World (TM)
http://www.bullopensource.org/ext4

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