Btrfs on zoned block devices

Posted Apr 19, 2021 23:18 UTC (Mon) by ikm (subscriber, #493)
Parent article: Btrfs on zoned block devices

Are files with the "nocow" attribute set supported? Aren't they overwritten in-place?

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Btrfs on zoned block devices

Posted Apr 20, 2021 10:49 UTC (Tue) by intelfx (subscriber, #130118) [Link] (3 responses)

> Are files with the "nocow" attribute set supported?

To my understanding, `NOCOW` in btrfs is akin to a hint and is honored on a best-effort basis. For example, you can create snapshots of NOCOW files normally.

Btrfs on zoned block devices

Posted Apr 20, 2021 11:04 UTC (Tue) by kdave (subscriber, #44472) [Link] (1 responses)

The nocow attribute is sticky and is not a hint. That snapshotting of nocow files write works because first write to a nocow file on the new snapshot will do one time cow, the original file is unaffected.

Btrfs on zoned block devices

Posted Apr 20, 2021 12:05 UTC (Tue) by intelfx (subscriber, #130118) [Link]

So, technically, `NOCOW` is violated that one time. OK, I assumed this principle expands to the rest of operations in btrfs.

Btrfs on zoned block devices

Posted Apr 20, 2021 22:23 UTC (Tue) by flussence (guest, #85566) [Link]

The flag means copying of a file should be done proactively and not on-demand, it doesn't preclude making copies outright. That would make it DRM (in the derogatory sense, not the graphics one).

Btrfs on zoned block devices

Posted Apr 20, 2021 11:01 UTC (Tue) by kdave (subscriber, #44472) [Link]

No, the NOCOW file attribute is not supported.