Proper UNIX FS, ISC disk performance <was: Disk benchmark (long)>

[Dick Dunn]

shwake at raysnec.UUCP (Ray Shwake) writes in response to some details of what
Shawn Hayes is looking for, namely...

> >...better disk performance capabilities( perhaps by
> >putting the inode and the file data at the same point on the disk) or another
> >way of accessing/updating the data that avoids the inode update penalty. 

> 	Ah, but then it wouldn't be a proper UNIX filesystem. In the latter,
> the inodes go in the superblock, while the data goes in the data block.

No, the inodes do not go in the superblock.  The superblock contains the
overall information for the file system.  Traditionally, the inodes go
in a separate area of the disk from the data--that is, there's a region of
inode blocks and a region of data blocks.  (Indirect blocks--those used for
describing the file's allocation beyond the first handful of blocks which
are listed in the inode--are taken out of the data block area.)

It is possible to carve up the file system so that you've got a chunk of
inodes, a chunk of data, another chunk of inodes, more data,... in an
attempt to get the inodes closer to the data they describe.  This is the
organization used in the BSD fast file system; the inode-data chunks are
called "cylinder groups."

It's an open question whether the cylinder groups provide any real advan-
tage.  They may be more work than they're worth nowdays (with seek times
being lower than in the past, but rotational latency still the same as long
ago).

> 	I've been quite impressed with Interactive's latest UNIX, which
> implements a Fast Filesystem (unlike the Berkeley version, ISC's *is*
> System V compatible)...

Yes, but one wouldn't expect a BSD file system to be Sys V compatible,
since the BSD FFS predates System V!  Also, of course, that's a market
issue:  ISC is in the Sys V market, and therefore spent some effort to stay
SysV compatible on disk; BSD (meaning "those vendors who sell BSD-based
systems" actually) isn't in the same market.

In the case of both BSD and ISC ffs'es, the major gains are from allocation
via bitmap and increasing the transfer size.
-- 
Dick Dunn     rcd at ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Never offend with style when you can offend with substance.