Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!sun-barr!newstop!sun!snafu!lm From: lm@snafu.Sun.COM (Larry McVoy) Newsgroups: comp.unix.admin Subject: Re: tunefs Message-ID: <141722@sun.Eng.Sun.COM> Date: 1 Sep 90 03:07:14 GMT References: <2697.26d4fadc@csc.anu.oz> <1592@shodha.dec.com> Sender: news@sun.Eng.Sun.COM Reply-To: lm@sun.UUCP (Larry McVoy) Organization: Sun Microsystems, Mountain View Lines: 32 In article emv@math.lsa.umich.edu (Edward Vielmetti) writes: > This changes the minimum amount of free space from the default (10 > percent) to a more adequate [in a disk-space-starved environment, like > ours :-(] 2% (that's the number 2 in the /tunefs commands). The fast file system (you have it if you have tunefs) has a multi alg allocator. The file system trys to allocate things nicely. For example, if you are running with a rot delay of 4ms (most people are) then the allocator will try to allocate the next block on the same track, 4ms done the line. Failing that, it will try anywhere on the same track, then anywhere in the same cylinder group, and finally a some sort of brute force search (I may have this a bit wrong; go read the paper to see what really happens). Anyway, the scoop is that as the file system fills up it gets less and less likely that the allocator can put things where you want. That's where the 10% free comes in. That 10% is evenly distributed across the disk (in theory) which makes it more likely that the allocator can put things in the right place. So, should you do it? If you have an absolutely static disk, mounted read only all the time, then crank that %free down to 0, fill up the disk and sleep easy. If you have an active disk (/tmp or user directories) then you should leave it at 10% if you want reasonable performance. Oh - another consideration: if all of your files are very small, less than the file system block size (usually 8K), then most of this doesn't matter and you can crank it down to 2% w/o too much trouble. --- Larry McVoy, Sun Microsystems (415) 336-7627 ...!sun!lm or lm@sun.com