COW and volume management? [LWN.net]

COW and volume management?

Posted Feb 5, 2026 13:26 UTC (Thu) by intelfx (subscriber, #130118)
In reply to: COW and volume management? by malmedal
Parent article: Filesystem medley: EROFS, NTFS, and XFS

> what? ZFS is a log-structured filesystem, like e.g. Sprite LFS and NetApp WAFL.

It's not a log-structured filesystem, it is a write-anywhere-*ish* (but not really) kind of filesystem with a log (journal) hanging off its side for performance.

Very different things.

The core concept underpinning DMU is still just a (potentially recursive) map of blocks. That is the "original Unix filesystem". It's just ext2 with (admittedly, a lot of) extra steps.

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COW and volume management?

Posted Feb 5, 2026 16:05 UTC (Thu) by malmedal (subscriber, #56172) [Link]

> it is a write-anywhere-*ish* (but not really)

ish? Fully I believe. The only exception to write anywhere I am aware of is the uberblock. IIRC you basically have two logs, one for the uberblock only and one for everything else.

I believe NetApp does the same thing, the root inode, just like ZFS's uberblock, is constrained to a few known locations to avoid having to read every byte on every disk on a cold start.

> (but not really) kind of filesystem with a log (journal) hanging off its side for performance.

No, both metadata and data are always written to a new location when updated. There is no update in place neither in ZFS nor in NetApp WAFL.