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by ajross 292 days ago
> 2 - No, metadata robustness absolutely varies across filesystems.

That's misunderstanding the subthread. The upthread point was about metadata atomicity in snapshots, not hardware corruption recovery. A filesystem like ZFS can make sure the journal is checkpointed atomically with the CoW snapshot moment, where dm obviously can't. And I pointed out this wasn't actually helpful because this is a problem that has to be solved above the filesystem, in databases and apps, because it's isomorphic to power loss (something that the filesystem can't prevent).
1 comments
nh2 292 days ago
I believe it is helpful because you can stop an app (such as a DB), FS-snapshot, and then e.g. rsync the snapshot or use any other file based backup tool, and this snapshot is fast and will be correct.

Doing the same with a block device snapshot is not so easy.

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ajross 291 days ago
Again, if your system is "incorrect" having been stopped and snapshotted like that, it is also unsafe vs. power loss, something ZFS cannot save you from. Power loss events are vastly more common than poorly checkpointed database[1] events.

[1] FWIW: every database worth being called a "database" has some level of robust journaling with checkpoints internally. I honestly don't know what software you're talking about specifically except to say that you're likely using it wrong.

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nh2 290 days ago
You are conflating Consistency and Durability in a way that is not necessary.

FS snapshotting can be useful to work on files on which fsync() is never called, but for which you need a consistent cross-file view nontheless while the system is online.

Another example is the case of sqlite with default settings as discussed recently (https://news.ycombinator.com/item?id=45005866), where the presence of a file determines whether or not transactions will be missing on next access. Because SQLite does not fsync the file's parent directory by default, transactions would be correctly recorded in by FS snapshotting, but lost in a block device snapshot. Your argument is correct that they would also be lost in power loss, but that does not matter for the fact that there exists a valid way to use that software in some situations where you care about consistency and not about power losses.

This is why having such a feature in file systems is useful.

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