Re: [!! SPAM] SAVEFD

[Eginhard Jaeger]

SAVEFD is not about speed of access to data on the CMS minidisk, it's about 
saving real storage:
- without SAVEFD each user accessing a common minidisk has his own copy 
  of the file directory
- with SAVEFD all users accessing that common minidisk share a common copy.

Since file directory entries are 64 Bytes each accessing a minidisk with 1000 
files requires about 64kB in a user's virtual storage. This means that if you 
had 1000 users accessing that minidisk concurrently about 64MB of main storage 
could be saved by using SAVEFD. 

Back in the years when SAVEFD was introduced, when IBM internal PROFS systems 
often had 1000-2000 concurrent users, and when storage was still expensive and 
correspondingly scarce, the use of SAVEFD often made the difference between 
good performance and absolutely catastrophic performance with the paging system 
thrashing and nobody getting anything useful done.

But on VM systems where the main load is caused by Linux guests and you have 
only a few dozen (or even hundred) CMS users you can forget about SAVEFD. All 
the more so if you stopped long ago thinking in MegaBytes when talking about 
storage ..

Eginhard Jaeger
 
- Original Message - 
  From: Gentry, Stephen 
  To: IBMVM@LISTSERV.UARK.EDU 
  Sent: Thursday, May 06, 2010 3:54 PM
  Subject: [!! SPAM] SAVEFD


  With the speed  of z9's and z10's  and the speed of DASD subsystems, is there 
much of a need to use SAVEFD anymore?

  I guess if a minidisk had extremely high utilization then it might.

  Thoughts?

  Thanks,

  Steve

Re: SAVEFD

[Moore, Terry A.]

Eginhard has nailed it.  I remember the performance boost we got back in
those days.  It was truely a difference between a useable and a
non-usable system, and it was all about storage constraints.
 
Terry



From: The IBM z/VM Operating System [mailto:ib...@listserv.uark.edu] On
Behalf Of Eginhard Jaeger
Sent: Friday, May 07, 2010 2:55 AM

SAVEFD is not about speed of access to data on the CMS minidisk, it's
about saving real storage:
- without SAVEFD each user accessing a common minidisk has his own copy 
  of the file directory
- with SAVEFD all users accessing that common minidisk share a common
copy.
 



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SAVEFD

[Gentry, Stephen]

With the speed  of z9's and z10's  and the speed of DASD subsystems, is
there much of a need to use SAVEFD anymore?

I guess if a minidisk had extremely high utilization then it might.

Thoughts?

Thanks,

Steve