By default cp and tar don't work well, but they have a --sparse or -S option.
I don't know if --sparse=auto works on NFS (it depends on the FS reporting
the real disk usage), but --sparse=always should work (never tested).
Ah, but cp --sparse will still read the holes, so it's pointless on NFS.
"Good job" means in their creation, right? Because rsync --sparse could also
save downloading the holes through the network.
My colleague, Dan, who helped me develop the script said:
"rsync --sparse" has the same deficiency as "cp --sparse": since there's
no dependable API for finding the holes in a sparse file over NFS,
copying the file from $HOME to /tmp requires reading all the zeros over
NFS regardless of what userland program is used to do the copying.
The key to how we speed up the process of creating and storing the COW
file is that the copy that's stored in the user's home directory is not
itself a sparse file: it's a tar archive which describes where the holes
in the file are. That is, we use something along the lines of
cd /tmp; /usr/bin/star -sparse -c -f $HOME/UML/cow_file.star
cow_file
to save the COW file, and
cd /tmp; /usr/bin/star -sparse -x -f $HOME/UML/cow_file.star
to unpack it again.
The actual script uses compression as well (i.e. the -bz flag) so the
files are a bit smaller than they would be otherwise. All in all, using
star in this manner make the process a whole lot faster than cp or rsync.
(GNU tar could also be used but we found that it was a little slower.)
regards,
bob
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Bob Manson Phone (416)978-5898
Systems Administrator, ECF Fax (416)978-7320
University of Toronto email [EMAIL PROTECTED]
Toronto, Canada M5S 1A4 or [EMAIL PROTECTED]
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