Re: ext2/ext3 different block_sizes/i_size/e2fsck question

[Evgeniy Ivanov <lolkaantimat@gmail.com>]

On Thu, Mar 25, 2010 at 4:55 AM,  <tytso@mit.edu> wrote:
> On Wed, Mar 24, 2010 at 10:27:24PM +0300, Evgeniy Ivanov wrote:
>>
>> Sorry for bothering list with my ext2 questions.
>> I got into trouble with my ext2 implementation and filesystem with
>> 1024 block size. Sometimes when I write files they're written
>> correctly (md5sum is the same as original, i_size is correct either),
>> but e2fsck changes i_size to another values (which breaks files). E.g.
>> 67445000->67446784 or 67445248->67446784. I see that new sizes are
>> numbers of multiples of 1024.
>> Strange thing is that I can't reproduce this problem with 2048 and
>> 4096 block sizes. I thought the problem was in trash in unused part of
>> last block (actually it is zeroed), but then it would be reproduceable
>> in fs with another block size.
>
> E2fsck will adjust i_size if it is smaller than the number of blocks
> than you have allocated.  So in the case of 67445000->67446784, your
> file probably had 65866 1k blocks, and since 67445000 is less than
> (655865*1024)+1, e2fsck assumed that your i_size was wrong, and so it
> asked for permission to fix it.
>
> Put another way, if you have 2 blocks in 1k file, and i_size is 1024,
> it clearly must be wrong.  If it's 1025, maybe we're only using 1 byte
> in the last block; but if i_size is less than or equal to 1024, then
> why was the 2nd block allocated in the file in the first place?

Thank you for your explanation.
My problem was in miscalculation of first triple indirect block. I
used following thing "triple_ind_s = doub_ind_s + pow(addr_in_block,
2)" and it was a bad idea to use pow() instead of multiplication or
shifting. It was ok with gcc (and libc), but caused a problem with ACK
(I get value of 1 less, thus each last double indirect block became a
hole instead of data). Since that was both in reading and writing md5
sums were correct (and in Linux I checked them only after e2fsck).
Funny bug :)


-- 
Evgeniy Ivanov
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