On Fri, 2008-03-14 at 22:37 -0600, Von Grant Fugal wrote: ... > Recently, I put two new drives in my mythtv box (which also has surfer > right now), an 80G and a 120G (which actually turn out to be 74G and > 112G). I decided to lvm the two drives into one big 186G (shark), and > as much as I wanted to use XFS, I knew I was entirely likely to want to > shrink it at some point. Hence I chose ext3. Upon doing so, just after > formatting shark to ext3, I noticed there were about 8G completely > gone from the getgo. That's what I call painful overhead. So right off > the bat that's about 20 hours of mythtv I don't have room for. :( This > is compared surfer which has virtually no such discrepency. > > total - (used + free) from the df command > xfs: 244076732 - (150157648 + 93919084) = 0 > ext3: 183847020 - (120945192 + 53562948) = 9338880 (about 5% overhead) > > Maybe xfs merely reports it's overhead as used space where ext just > hides it, who knows. I _do_ know that I would have gawked at nearly 12G > being used when I first formated surfer, and I remember no such event, > so it's at least a smaller overhead.
Actually that makes a lot of sense. You see Ext3 reserves (by default) 5% for root. This doesn't show up with df. This is so that if a user runs the space and has no more room, root is still able to log in and take care of the problem. It will also allow daemons to continue to run as root. Making the system still semi usable. IIRC, it dates back to ext2 days but I could be wrong. The other thing to remember is ext3 writes out all the superblocks and inodes when it is formatted. XFS and others write them on the fly as files are created. Nathan
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