On Monday 21 November 2005 22:20, Bob Manson wrote: > > 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.
Ok, fully correct. Indeed, I think there's in general little point in using rsync over mounted shares - it will treat the file as locally present and freely read from it. > 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.) Ok, very interesting - thanks for the thorough explaination. -- Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!". Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894) http://www.user-mode-linux.org/~blaisorblade ___________________________________ Yahoo! Mail: gratis 1GB per i messaggi e allegati da 10MB http://mail.yahoo.it ------------------------------------------------------- This SF.Net email is sponsored by the JBoss Inc. Get Certified Today Register for a JBoss Training Course. Free Certification Exam for All Training Attendees Through End of 2005. For more info visit: http://ads.osdn.com/?ad_id=7628&alloc_id=16845&op=click _______________________________________________ User-mode-linux-user mailing list User-mode-linux-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/user-mode-linux-user
