Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!apple.com!goldman
From: goldman@apple.com (Phil Goldman)
Newsgroups: comp.sys.mac
Subject: Re: System 7.0 Virtual Memory Question
Message-ID: <2108@internal.Apple.COM>
Date: 27 May 89 01:11:33 GMT
Sender: usenet@Apple.COM
Organization: Apple Computer
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References:<758@cf-cm.UUCP> <653@bridge2.ESD.3Com.COM> <31723@apple.Apple.COM> <8054@batcomputer.tn.cornell.edu>

In article <8054@batcomputer.tn.cornell.edu> 
mha@batcomputer.tn.cornell.edu (Mark H. Anbinder) writes:
> Does Apple's VM at least know how to take advantage of contiguous space,
> and will Apple recommend that users provide contiguous space?  Having a
> contiguous chunk of space for a virtual memory system to play in speeds
> it up enormously.  When the Virtual program from Connectix creates its
> virtual memory storage on disk, it grabs eight megs of contiguous space
> and KEEPS it.  I hope Apple's will do the same.

Using contiguous disk space will not speed up paging time in most cases,  
as long as the file is not *seriously* fragmented (which is rare unless 
the disk is very close to full).  The odds of a particular page lying in 
multiple file extents is very low (or non-existent if the page size is at 
least as small as the min extent  size).  Also, due to locality pages that 
need to be read in should be in the same extent.  Finally, when the paging 
gets hot and heavy pages will be reclaimed and often written out to an 
entirely different part of the file, so big seeks will happen anyway.

Of course, most of these arguments depend on the fact that the VM backing  
store file is a sizeable chunk of the entire disk.  Otherwise, the disk  
seeks may become very costly if the extents are physically far apart.

All this said, space will be allocated contiguously if at all possible,  
probably because it's the easiest way to implement it.  The file will not
need to be created each time the machine boots either.

Also, you don't have to reformat your disk to get it.

-Phil Goldman
Apple Computer