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DM snapshot provides the COW block feature and it will match your idea
since the size of COW device is much smaller genearally. But it doesn't
support off-line upgrade either. If you do, it is equivalent to corrupt
filesystem for DM snapshot device.
Here is pros/cons of DM snapshot comparing a union.
- the number of bytes to be copied between devices is much smaller.
- the type of filesystem must be one and only.
- the fs must be writable, no readonly fs, even for the lower original
device. so the compression fs will not be usable. but if we use
loopback mount, we may address this issue.
for instance,
mount /cdrom/squashfs.img /sq
losetup /sq/ext2.img
losetup /somewhere/cow
dmsetup "snapshot /dev/loop0 /dev/loop1 ..."
- it will be difficult (or needs more operations) to extract the
difference between the original device and COW.
- DM snapshot-merge may help a lot when users try merging. in the
fs-layer union, users will use rsync(1).
- in fs-based union, users can add/remove members(layer) dynamicall
without unmounting. of course, all files on the removing layer should
not be busy.
Also here is my concern about UnionMount. All these issues have been
reported before.
- for users, the inode number may change silently. eg. copy-up.
- link(2) may break by copy-up.
- read(2) may get an obsoleted filedata (fstat(2) too).
- fcntl(F_SETLK) may be broken by copy-up.
- unnecessary copy-up may happen, for example mmap(MAP_PRIVATE) after
open(O_RDWR).
J. R. Okajima
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