I think I have a misconception of what copy on write in btrfs means for
individual files.
I had originally thought that I could create a large file:
time dd if=/dev/zero of=10G bs=1G count=10
10+0 records in
10+0 records out
10737418240 bytes (11 GB) copied, 100.071 s, 107 MB/s
real 1m41.082s
user 0m0.000s
sys 0m7.792s
Then if I copied this file no blocks would be copied until they are
written. Hence the two files would use the same blocks underneath. But
specifically that copy would be fast. Since it would only need to write
some metadata. But when I copy the file:
time cp 10G 10G2
real 3m38.790s
user 0m0.124s
sys 0m10.709s
Oddly enough it actually takes longer then the initial file creation.
So I am guessing that the long duration copy of the file is expected and
that is not one of the virtues of btrfs copy on write. Does that sound
right?
I was looking at a virtual machine solution and thought btrfs would be
great if I could copy the vm disk to a new file at low cost and then
launch that vm and customize it to my needs.
OS Ubuntu 12.10
Mike Power
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