Second Block on Partition overwritten with 0xFF

[Tomas Pospisek's Mailing Lists]

On Thu, 6 Sep 2007, Andreas Dilger wrote:

> On Sep 06, 2007  23:02 +0200, Tomas Pospisek's Mailing Lists wrote:
>> On Thu, 6 Sep 2007, Christian Kujau wrote:
>>> On Thu, 6 Sep 2007, Tomas Pospisek ML wrote:
>>>> default) at 0x400. Thus as I understand it, it *would* be possible for
>>>> the ext3 driver to pysically write to those first sectors inside its
>>>> partition.                                                ^^^^^^
>>>
>>> Yes, ext3 will write *inside* its assigned partition, but not outside.
>>
>> Thanks, however it seems I can not get through what I need to know -
>> sorry for that. I *do* know that ext3 will only write to its partition
>> only. But once mke2fs has run:
>>
>> * will ext2/3 *ever* write to the first 4 sectors on *its* partition?
>>
>> Same question restated: is it possible that ext2/3 will write into the
>> space before the first block group [1]?
>
> The ext2/3/4 superblock is at offset 1024 bytes.  It is written by marking
> the buffer it is in dirty.  If the filesystem blocksize is > 1024 bytes
> then the whole block will be written to disk (including the first sectors).
>
> That said, the buffer cache is coherent when written by the filesystem and
> when written via /dev/XXX so any modifications made to the first sectors
> should be rewritten each time the superblock is marked dirty.  The ext3
> code will never itself modify those sectors.

I just remembered, that once the problem occured when there was very high 
memory pressure. I.e. the OOM killer went around and killed applications, 
the machine rebooted, at which point the FS was broken.

So a naive ad hoc theory of mine for the FS corruption would be that the 
FS was unmounted at a moment when processes wouldn't receive any more 
memory from the OS (due to OOM) and thus umount would flush/write out the 
first block (I believe it needs to obligatorily clear the dirty FS flag at 
umount) which it failed to properly allocate before?!?
*t

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   Tomas Pospisek
   http://sourcepole.com -  Linux & Open Source Solutions
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