ext4 and data loss
Posted Mar 13, 2009 15:26 UTC (Fri) by iabervon
In reply to: ext4 and data loss
Parent article: ext4 and data loss
In an ideal world, that would be exactly what you'd see: after a cold restart, the system would come up in some state the system was in at a time close to the crash, not some made-up non-existent state the filesystem cobbles together from bits of wreckage.
The model works if you include the fact that, in a system crash, unintended things are, by definition, happening. Any failure of the filesystem to make up a possible state afterwards appears as fallout from the crash. Maybe some memory corruption changed your file descriptors, and your successful writes and successful close were some other file (but the subsequent rename found the original names). Maybe something managed to write zeros over your file lengths. It's not a matter of standards how often undefined behavior leads to noticeable problems, but it is a matter of quality.
fsync has nothing to do with what other processes see. fsync only forces a write to stable storage; it has no effect on the filesystem as seen from a running system. In your terminology, it just forces the conceptual "filesystem" process to take a snapshot at that instant.
That's what I meant to say: it makes the "filesystem" process see everything that had already happened. (And, by extension, processes that run after the system restarts, looking at the filesystem recovered from stable storage)
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