From owner-freebsd-questions  Fri Sep 15  5:18:33 2000
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Date: Fri, 15 Sep 2000 14:18:28 +0200 (CEST)
From: Konrad Heuer <kheuer@gwdu60.gwdg.de>
To: Mark Ovens <marko@FreeBSD.ORG>
Cc: Greg Lehey <grog@lemis.com>, Dan Nelson <dnelson@emsphone.com>,
	Bruce Petro <bpetro@usa.com>,
	freebsd-questions <freebsd-questions@FreeBSD.ORG>
Subject: Re: Compare FBSD File System
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On Fri, 15 Sep 2000, Mark Ovens wrote:

> On Fri, Sep 15, 2000 at 11:26:18AM +0930, Greg Lehey wrote:
> > On Thursday, 14 September 2000 at 14:04:02 -0500, Dan Nelson wrote:
> > > In the last episode (Sep 14), Bruce Petro said:
> > >> Could someone shoot back a brief review of how the FBSD file system
> > >> compares to the old FAT system? General information is fine, but
> > >> please also cover:
>
> [......]
>=20
> Are the blocks which are used "by frag" in a special reserved area of
> the disk or can any block be used for this purpose as and when
> required and, if so, how does the system keep track of which frags in
> a block are in use? Is it meta-data stored in the directory entries or
> in the inode table perhaps?

An entire filesystem is divided into cylinder groups which contain a
backup copy of the super block which describes the filesystem, inode
blocks, bitmap blocks and data blocks.

The system keeps track of used fragments and blocks by bitmap patterns in
the bitmap blocks; the use of bitmaps gives better performance than using
linked lists etc. would give.

Fragments can be stored anywhere in the data block area.

The use of cylinder groups increases perfomance since inode and data
block(s) of files are in neighbourhood and can be read with little
movement of the disk heads. The system tries to store larger files in
consecutive data blocks to make read ahead possible. If files grow too
large these clusters of data blocks are distributed over different
cylinder groups to prevent a single cyclinder group from getting full
while others have much free space.

Regards

Konrad Heuer                                    Personal Bookmarks:

Gesellschaft f=FCr wissenschaftliche
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rg
Deutschland (Germany)

kheuer@gwdu60.gwdg.de



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