[donatj]

> what does ASIF do that Qcow2 doesn't

Mount natively in macOS

> why does it take so long to copy [...] out of a dmg

Compression mostly. DMG contents can optionally be compressed using zlib, lzfse, or slow as molasses bzip2.

Also Gatekeeper.

[1e1a]

Additionally, while I don't know much about APFS, I don't think it would be beneficial to point the extracted app to blocks that are also part of the dmg file, i.e. some copying has to happen anyway.

[fragmede]

in a perfect theoretical filesystem, copy-on-write means copying is as cheap as moving a file, though uncompressing time makes sense.

[xoa]

A perfect theoretical filesystem can still have subjective user configurable choices though right? Like case sensitivity, UTF normalization, checksum hash function, extra copies of data/metadata to store for redundancy/healing, etc (as well as compression/encryption). I think ZFS is a pretty strong real world example of a CoW FS, but you can still set a lot of different properties between sub-fs and then need to copy when you go between them to get the structural changes.

Disk images are supposed to function as if they're attached storage I think, and have different properties from what FS you're running on boot or your home folder (which themselves can be different, I run my home folder on my main Mac off a NAS via iSCSI). I'm not sure any underlying FS would avoid a copy operation there in general?

[galad87]

Of course, but that works only if the files are already in the same partition. A dmg is a virtual image, even if it's stored in the same partition, once mounted it acts like another partition.