Space savings should use COW

Posted Sep 2, 2004 13:23 UTC (Thu) by utoddl (subscriber, #1232) [Link]

I have a script that makes good use of hard links -- not for space saving so much as time saving, but it saves a lot of space as well. I keep a copy of my RedHat/Fedora/whatever ISO images, and occasionally use wget to grab all the updates into another directory. These updates contain all 19 gazillion versions of the updated packages -- way more than will fit on a CD -- when what I really want of course is the latest version of each. So I use cpio to make a hard link duplicate tree of all those updates (i.e. real directories, hard linked files). That's pretty quick, 'cause it's not moving any data, just creating dir entries. Then my script throws out everything I don't want from that tree -- all the older versions of a given rpm -- and I'm left with a small enough set of rpms to fit onto a CD. I add my own favorite goodies that aren't on the distro (config files, utilities, etc.), and burn an ISO from that. That gives me the original distro CDs plus an extra CD with all current updates and my favorites all on CDs I can carry around so I can install on and update the various boxes I play with at home, work, friends' and family's houses, wherever. (Heck, I'll stick a copy of the scripts here if anybody wants to play with 'em.)

Using hardlinks for this was a natural. Having said that, I recall only using hardlinks once before, a long time ago, and that was specifically for space savings.

Posted Sep 10, 2004 0:46 UTC (Fri) by roelofs (guest, #2599) [Link]

So do you (or anybody else for that part) know of any other uses for hard symlinks that don't have anything to do with space savings?

Assuming you really meant "hard links," I use them to avoid accidentally deleting local mail files. Since such files get updated every day, usually several times a day, even daily incremental backups aren't sufficient to recover from an accidental deletion. But with hard-linked copies in a separate directory (e.g., ../.backups/foo.backup, etc.), you're safe. (And when you really do want to nuke the file, just truncate it to zero bytes--I wrote a trivial "trunc" utility that simply uses truncate() or ftruncate() for this purpose.) Of course, I suppose I could simply keep the "real" copy in the same hidden directory and use local symlinks to append to it...but with hard links you save one letter in every ls(1) command (i.e., -L). :-)

I've also occasionally used hard links to save a temporary MPEG or PDF file downloaded by Netscape; under some conditions, the temporary file will disappear as soon as the download is complete, but if you make a hard link at any point prior to that, the link will remain. For multi-megabyte downloads, that can be convenient, albeit not absolutely critical...

Greg