ext4 and data loss
Posted Mar 13, 2009 0:16 UTC (Fri) by quotemstr
In reply to: ext4 and data loss
Parent article: ext4 and data loss
That's from the filesystem's perspective.
From the application's perspective, the entire sequence of "atomically replace the content of file A" failed -- file A was left in an indeterminate state. The application has no way of stating that it wants that replacement to occur in the future, but be atomic, except to use
open-write-close-rename. The filesystem should ensure that the entire operation happens atomically, which means flushing the file-to-be-renamed's data blocks before the rename record is written.
What the application obviously meant to happen is for the filesystem to commit both the data blocks and the rename as some point in the future, but to always do it in that order. Atomic rename without that guarantee is far less useful, and explicit syncing all the time will kill performance.
These semantics are safe and useful! They don't impact performance much because the applications that need the fastest block allocated -- databases and such -- already turn off as much caching as possible and do that work internally.
Atomic-in-the-future commits may go beyond a narrow reading of POSIX, but that's not a bad thing. Are you saying that we cannot improve on POSIX?
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