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Well yeah, because it's supposed to replace those lower levels.
Also, BTRFS does provide advantages over ext4, such as snapshots, which I think are fantastic since I can recover if things go sideways. I don't know what your use-case is, so I don't know if the features BTRFS provides would be valuable to you.
Generally, if a lower level can do a thing, I prefer to have the lower level do it. It's not really a reason, just a rule of thumb. I like to think that the lower level is more efficient to do the thing.
I use LVM snapshots to do my backups. I don't have any other reason for it.
That all being said, I'm using btrfs on one system and if I really like it, I may migrate to it. It does seem a whole lot simpler to have one thing to learn than all the layers.
Yup, I used to use LVM, but the two big NAS filesystems have a ton of nice features and they expect to control the disk management. I looked into BTRFS and ZFS, and since BTRFS is native to Linux (some of my SW doesn't support BSD) and I don't need anything other than RAID mirror, that's what I picked.
I used LVM at work for simple RAID 0 systems where long term uptime was crucial and hardware swaps wouldn't likely happen (these were treated like IOT devices), and snapshots weren't important. It works well. But if you want extra features (file-level snapshots, compression, volume quotas, etc), BTRFS and ZFS make that way easier.
I am interested in compression. I may give it a try when I swap out my desktop system. I did try btrfs in it's early, post alpha stage, but found that the support was not ready yet. I think I had a VM system that complained. It is older now and more mature and maybe it's worth another look.
Actually, the lower level may likely be less efficient, due to being oblivious about the nature of the data.
For example, a traditional RAID1 mirror on creation immediately starts a rebuild across all the potential data capacity of the storage, without a single byte of actual data written. So you spend an entire drive wipe making "don't care" bytes redundant.
Similarly, for snapshotting, it can only track dirty blocks. So you replace uninitialized data that means nothing with actual data, the snapshot layer is compelled to back up that unitiialized data, because it has no idea whether the blocks replaced were uninialized junk or real stuff.
There's some mechanisms in theory and in practice to convey a bit of context to the block layer, but broadly speaking by virtue of being a mostly oblivious block level, you have to resort to the most naive and often inefficient approaches.
That said, block capacity is cheap, and doing things at the block level can be done in a 'dumb' way, which may be easier for an implementation to get right, versus a more clever approach with a bigger surface for mistakes.
Those are some good points. I guess I was thinking about the hardware. At least where I do RAID, it's on the controller, so that offloads much of the parity checking and such to the controller and not the CPU. It's all probably negligible for the apps that I run, but my hardware is quite old, so maybe trying to squeeze all the performance I can is a worthwhile activity.