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ext4 and data loss

ext4 and data loss

Posted Mar 13, 2009 16:04 UTC (Fri) by giraffedata (subscriber, #1954)
In reply to: ext4 and data loss by quotemstr
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. Most filesystems weaken this guarantee somewhat, but leaving NULL-filled and zero-length files that never actually existed on the running system is just unacceptable.

You mean undesirable. It's obviously acceptable because you and most your peers accept it every day. Even ext3 comes back after a crash with the filesystem in a state it was not in at any instant before the crash. The article points out that it does so to a lesser degree than some other filesystem types because of the 5 second flush interval instead of the more normal 30 (I think) and because two particular kinds of updates are serialized with respect to each other.

And since you said "system" instead of "filesystem", you have to admit that gigabytes of state are different after every reboot. All the processes have lost their stack variables, for instance. Knowing this, applications write their main memory to files occasionally. Knowing that even that data isn't perfectly stable, some of them also fsync now and then. Knowing that even that isn't perfectly stable, some go further and take backups and such.

It's all a matter of where you draw the line -- what you're willing to trade.

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