why separate filesystems?

[David S. Herron]

In article <1990Aug24.091111.508 at bbt.se> pgd at bbt.se (P.Garbha) writes:
>In article <1053 at p4tustin.UUCP> carl at p4tustin.UUCP (Carl W. Bergerson) writes:
>>Performance:
>>
>>	"Smaller filesystems are faster" - Xenix Installation Guide
>>
>>	This is generally true for all versions of *ix.
>
>Can you explain why? Becuase I cannot see why it should be like that.

Depends on activity in your system.  (Your mileage will vary depending
on road conditions and the like)

Like you surmise, head-motion is the reason.  And in this case more
isn't merrier.

Within a file system the head will be going back and forth between
the group of inodes and the data-blocks.  For large files, especially,
it will be popping around a lot as you get into the extended blocks
of the file.

With a smaller file system the distance between the inodes and
data blocks becomes less important.  This still depends on a lot
of "other factors".

Like it works really well if most of your file activity centers
itself on partition at a time.

For instance, during a compile the head'll roam between /tmp, the
swap device, and / (for the libraries) or /usr (for /usr/lib
libraries and /usr/include files).  If you put these partitions
close together you get some benefits from the heads not moving
around a lot.  I expect that putting <swap> and /tmp right next to
each other is a Big Win.  Especially since they both tend to be
fairly small.

Somewhere I remember reading (of) a study which claimed that the
old file system performance max'd out around a 70 meg partition.
This should mean that Usenet on any SysV machine is gonna be a
whooooole lot slower than it should.  (Nowaday's serious news partitions
are well above 200 meg's)



One of the good features of the BSD Fast File System is that they
scattered inodes around the disk and used some heuristics to induce
data blocks for a file to be in the same "cluster" which holds the
inode.  Thus if you're heavily using a particular file then the
disk head's will tend to stay in this one fairly small area.



Note that on my development system I have only one partition because
it's "only" a 100 meg disk and I needed to fit an X11R3 on it along
with a whooooole lot of networking software.  (OSI isn't small..  ;-)
at least at this stage..)
-- 
<- David Herron, an MMDF & WIN/MHS guy, <david at twg.com>
<- Formerly: David Herron -- NonResident E-Mail Hack <david at ms.uky.edu>
<-
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