Hello,
With backed up files consisting of hard links, I usually use dd to copy
the file systems at the block level
# dd if=/dev/sda of=/dev/sdb bs=20M
and then expand the file system. This is because I found that tools like
rsync, while usually fast, are extremely slow when dealing with millions
of hard linked files.
For me, I had to copy over BackupPC hardlinked files from a full disk to
a smaller disk, both using ext4, and I could not use dd. What normally
should have taken an hour, instead took almost a week. (Yes, I wanted to
use btrfs, but it had a hard link limit of 255 - don't know if it still
does.)
It would be nice to have a btrfs command that could rapidly copy over
the file system, snapshots, and all other file system info.
But what benefit would having a native btrfs 'copy/rsync' command have
over the dd/resize option?
Pros
- Files will be immediately checksumed on new disks, but this may not be
as important since a checksum/verify command will be implemented.
- Great 'feature' for copying files to new drives, and keeping
snapshots. Could even be used to export snapshots.
- I believe compressed files will have to be uncompressed and
recompressed, depending on when file is checksummed. (I may be wrong on
this one). This will actually be a con for slow and/or high load machines.
- One command instead of many (dd -> resize -> verify).
Cons
- File system would still have to be unmounted, or at least read-only,
as I doubt the command will have rsync's update or delete abilities.
But, maybe it could.
Questionable
- May be faster than dd/resize, or it may be just as slow as rsync is
with hard links. And I am talking about dozens to thousands of
snapshots, and millions to billions of files.
Matt
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