From owner-freebsd-questions Thu May 23 11:11:53 1996 Return-Path: owner-questions Received: (from root@localhost) by freefall.freebsd.org (8.7.3/8.7.3) id LAA06406 for questions-outgoing; Thu, 23 May 1996 11:11:53 -0700 (PDT) Received: from phaeton.artisoft.com (phaeton.Artisoft.COM [198.17.250.211]) by freefall.freebsd.org (8.7.3/8.7.3) with SMTP id LAA06401 for ; Thu, 23 May 1996 11:11:50 -0700 (PDT) Received: (from terry@localhost) by phaeton.artisoft.com (8.6.11/8.6.9) id LAA07821; Thu, 23 May 1996 11:04:23 -0700 From: Terry Lambert Message-Id: <199605231804.LAA07821@phaeton.artisoft.com> Subject: Re: Missing Disk Space (df -k) To: scrappy@ki.net (Marc G. Fournier) Date: Thu, 23 May 1996 11:04:23 -0700 (MST) Cc: sdd@ccd.tas.gov.au, freebsd-questions@freebsd.org In-Reply-To: from "Marc G. Fournier" at May 23, 96 01:16:50 am X-Mailer: ELM [version 2.4 PL24] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-questions@freebsd.org X-Loop: FreeBSD.org Precedence: bulk > > Okay.. Where has my missing 700Mbytes gone ? > > > > Anyone got any ideas.. Or is this just a symptom of a sick file store? > > > > Approximately 10% of a file system is reserved (set aside) for > fragmentation algorithms. That 10% *is* writable by root, but by > no other, so you can reach a negative value on it. It's 8%, with a recommended *minimum* of 5%, worst case. The reason for the reserve is because you are effectively hashing blocks onto the disk; according to Knuth (Sorting and Searching), the hash efficiency starts dropping off logarithmically at 85%. 10% was a good middle ground for a limit on degradation of hash efficiency. 8% makes people with *big* disks happy, of course, but doesn't change the mathematical principle involved. Any fill over a real 85% will start degrading performance, and any fill over 90% will probably degrade performance unacceptably. Actually, one could consider a change to the clustering algorithm to reduce the reserve by the average blocks in a cluster as a scaling factor. That is, if you didn't allow non-clustered blocks to be spread out all over (interfering with clustering) you could potentially divide the reserve by the cluster size. Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers.