Re: ext4 data=journal deprecation: long-standing industrial use case

[Zhang Yi <yizhang089@gmail.com>]

On 10/23/2026 7:22 AM, Andreas Dilger wrote:
> On Oct 19, 2026, at 22:18, Korber, Markus <markus.korber@anton-paar.com> wrote:
>>
>> On Oct 19, 2026, Andreas Dilger wrote:
>>
>>> are you running with delalloc disabled when testing ordered mode?
>>> That might be the source of NULs in recently written files.
>>
>> No, we did not explicitly disable delayed allocation.
>>
>> The data=ordered tests were run with:
>>
>>    data=ordered,commit=2,noatime,errors=continue
>>
>> so delayed allocation should have been enabled with the normal ext4
>> default.
> 
> What I mean is that if you *do* disable delalloc then this might stop
> the appearance of NULs at the end of a file after a crash.  The normal
> "ordered" mode writes should sync data to the filesystem before the
> size is updated in the inode, but delalloc does not do that.
> 
> Cheers, Andreas

I think this is caused by the default dioread_nolock behavior. Once
dioread_nolock is enabled, the extents allocated during writeback
are unwritten, so ordered mode doesn't take effect, and the
transaction that updates i_disksize may hit the disk before the
data I/O is persisted.

Cheers,
Yi.