ext4 and data consistency
Posted May 15, 2010 9:19 UTC (Sat) by anton
In reply to: ext4 and data consistency
Parent article: The Next3 filesystem
Speed is too important
For whom? For me data consistency is much more important. Before
barriers were supported, we ran ext3 on IDE disks without write
caching, and that's really slow. The file system was still fast
Stephen Tweedie didn't make data=journal the default, either.
Actually he did, at least at the start. Later it got changed (by
whom?) to data=ordered; that still has the potential to provide data
consistency unless existing files are overwritten.
As for an example: Consider a process writing file A and then file
B. With ext4 I expect that it can happen that after recovery B is
present and A is not or is empty. With ext3 I expect that this does
not happen. But given that I did not find any documented guarantees
in Documentation/filesystems/ext3.fs, maybe we should lump ext3 with
Still, my search brought up a Linux file system that gives
guarantees: In nilfs2.txt it says:
order=strict Apply strict in-order semantics that preserves sequence
of all file operations including overwriting of data
blocks. That means, it is guaranteed that no
overtaking of events occurs in the recovered file
system after a crash.
Yes, that's exactly the guarantee I want to see. This means that any
application that keeps its files consistent as visible from other
processes will also have consistent files after an OS crash.
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