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Not againNot againPosted Dec 31, 2010 1:00 UTC (Fri) by Nick (guest, #15060)In reply to: Not again by man_ls Parent article: Ext4 filesystem hits Android, no need to fear data loss (ars technica)
If you don't need files to be durable, then don't fsync them!
If there is a crash, then you don't have to bother with atomicity, just delete the files (which is possible because you didn't require durability).
These aren't inefficiencies, they are actually efficiencies, because they expose an API that is simple and efficient to implement for the memory management and filesystems. You can build arbitrarily more complex protocols on top of that.
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And yet again here we are Posted Dec 31, 2010 14:08 UTC (Fri) by man_ls (subscriber, #15091) [Link] What I want is not a durable file, but an atomic rename. Not an empty file, but either the old version or the new.Apparently the atomic rename is too confusing, or I am not explaining the use case clearly. Think about atomic appends to files then. When you append some sectors atomically, you want either the old file without the new sectors or the file with the new sectors at the end. What XFS did (and I experienced first hand) was add some uninitialized sectors to the file, and afterwards write the new content to the sectors. This behavior is apparently allowed by POSIX, and yet extremely annoying to users, who moved to other filesystems in droves. An atomic append would guarantee that the new sectors would either be present with the expected contents or not present at all, but never contain random garbage. We don't need an fsync after every sector, nor would it solve the problem: after the write but before the fsync the filesystem would be in an incoherent state anyway, albeit for a short time.
And yet again here we are Posted Jan 5, 2011 12:03 UTC (Wed) by nye (guest, #51576) [Link]
>What XFS did (and I experienced first hand) was add some uninitialized sectors to the file, and afterwards write the new content to the sectors. This behavior is apparently allowed by POSIX, and yet extremely annoying to users, who moved to other filesystems in droves.
To be fair there was more wrong with XFS than that - it could also corrupt *entirely unrelated* files. Personally I stopped using it when a power cut trashed /etc/passwd (and presumably a load of other files that were less obvious) despite nothing having it open at the time.
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