Initial Hammer2 filesystem implementation

Re:Initial Hammer2 filesystem implementation by takeda on Monday August 21, 2017 @11:23PM (#15069466)

I'm glad Hammer FS exist, it is always great to have options, and new filesystems have more to build on.

But this one seems weird to me:

> copy-on-write, implying snapshots and such, like ZFS, but snapshots are writable.

snapshot by definition should be read-only, also ZFS allows to create a new filesystem from snapshot (through zfs clone) which is also O(1) operation.

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Re:Initial Hammer2 filesystem implementation by antongribok on Tuesday August 22, 2017 @01:02AM (#15069847)

Read-only and read-write snapshots are possible with LVM, ZFS, Btrfs, and many commercial storage solutions.

One reason why you would want a writable snapshot is during backup validation. An application may need to do crash recovery before being able to read data, and having a writable filesystem makes things a lot easier.

I know that ZFS and LVM both send writes to a separate area, so the original snapshot is never modified (don't know about Btrfs implementation details).

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Re:Initial Hammer2 filesystem implementation by takeda on Tuesday August 22, 2017 @05:07PM (#15076402)

Yes, what I said in ZFS you can instantly (and without using extra space) create a writable filesystem from snapshot using zfs clone (you can create multiple filesystems from one snapshot), but snapshots by itself are always read only.

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Re:Initial Hammer2 filesystem implementation by Someone on Monday August 21, 2017 @11:34PM (#15069511)

That looks a bit like git versus svn. So, it _might_ be an insane idea that ends up being a good idea.

Especially if the default is to have a read only snapshot, I do not rule that out.

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