From: IanPercival [IanPercival@email.msn.com]
Sent: Friday, June 02, 2000 2:11 PM
To: Info-VAX@Mvb.Saic.Com
Subject: Re: VMS File Caching Futures (Was: Re: Andrew whatever)

I don't read the news group very often as I'm usually busy doing other
things - but there does seem to be an awful lot of discussion about VMS and
file data caching.   I don't particularly want to get involved in any of the
various discussions  - but there are an awful lot of misconceptions being
bandied about.  Feel free to email me at ian.percival@compaq.com if you have
serious questions or discussion.

VMS has two sets of data caching.
1. RMS. It can use Local or Global Buffers.  Some manual setiing and control
is required in order to use these if the application isn't using them
automatically.

2. System wide data caches.
First of all I agree that the system wide central data cache that VMS
has/had known variously as VCC or VIOC which was first implemented on VAX
V6.0 a number of years ago, is pretty poor by many performance metrics.  It
actually got worse, as functionality such as dynamic memory sizing got
dropped during the port to Alpha.  This is the reason there are a myriad
third party implementations of caches (OK maybe not quite that many - but
quite a few!) all of which worked better than VCC.

A new system wide file data cache called XFC (stands for extended file
cache) has been developed.   It is a true 64 bit cache - can store Tb of
data if you have the memory.
It can store over 100 CLOSED files (the VCC limitation was that it would
only store up to 100 closed files - not files in total!). XFC has no
limitation on number of closed files it will store - apart from the obvious
one of memory usage.
It can cache I/Os larger than 34 blocks (VCC was hard limited to 34)
It can perform readahead where appropriate.
It provides performance statistics such as absolute I/O response times in
microseconds.
It is dynamic or static or both in terms of memory usage.
My Alpha boots much quicker when using it! Many benchmarks are significantly
better when using it.

XFC V1.0 is about to enter the field test cycle.  By the time it is
released, it should have some other major performance features added -
making it perform even better for users of medium to large machines.

Hope this helps a bit!

Ian Percival
XFC Project Leader
OpenVMS Engineering
(writing from home on my son's birthday!)