Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!apple!usc!wuarchive!uunet!mcsun!ukc!dcl-cs!aber-cs!athene!pcg From: pcg@cs.aber.ac.uk (Piercarlo Grandi) Newsgroups: comp.unix.sysv386 Subject: Re: File system performance Message-ID: Date: 12 Nov 90 19:04:02 GMT References: <294@audfax.audiofax.com> <1990Nov3.124110.2155@metro.ucc.su.OZ.AU> <1990Nov3.222929.2164@servalan.uucp> <1990Nov5.222733.11408@unixland.uucp> <8257@gollum.twg.com> Sender: pcg@aber-cs.UUCP Organization: Coleg Prifysgol Cymru Lines: 34 Nntp-Posting-Host: odin In-reply-to: david@twg.com's message of 10 Nov 90 21:43:05 GMT On 10 Nov 90 21:43:05 GMT, david@twg.com (David S. Herron) said: [ ... on the BSD FFS and its ESIX incarnation ... ] david> That 10% limit is a heuristic invented when Berkeley invented FFS david> designed to help keep fragmentation down. No, it is designed to prevent the hashed quadratic search for a cylinder group with free blocks to be repeated too many times. It has *nothing* to do with fragmentation, only with the distribution of free blocks. [ ... on defragmenting the BSD FFS online ... ] david> Theoretically, yes. It would work best if the disk were david> `unmounted' first which is easiest to do if the system were david> brought to single user. It would also require writing software david> which would first sort the free list, then reorder all the data david> blocks into as contiguous an order as possible. The method is david> left as an excercise to the reader. But it would be pointless. The BSD FFS already keeps the blocks in nearly optimal order, without rewuiring reorganization. I doubt very much that in a FFS partition with sufficient free space (>10%) reorganization would buy a lot of extra thruput, whether the reoganization is online or offline. Moreover again: the issue we are discussing here is *scattering*, not fragmentation. In the BSD FFS fragmentation is the splitting of large blocks into small blocks for file tails. -- Piercarlo Grandi | ARPA: pcg%uk.ac.aber.cs@nsfnet-relay.ac.uk Dept of CS, UCW Aberystwyth | UUCP: ...!mcsun!ukc!aber-cs!pcg Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk