ADSM-L

Re: Optical Libraries and ADSM

1996-09-13 01:05:05
Subject: Re: Optical Libraries and ADSM
From: "Paul C. Beck" <beckpau AT IBM DOT NET>
Date: Fri, 13 Sep 1996 00:05:05 EST
** Reply to note from "ADSM: Dist Stor Manager" <ADSM-L AT VM.MARIST DOT EDU>   
      Thu, 12 Sep 1996 11:29:19 -0400

>> For those in the know, does ADSM treat these disks as truly random access
>> volumes?  I mean, it does treat them as a type of disk, not as sequencial
>> media or something equally strange..

   We are using 'unsupported' optical libraries - that is there is not native 
ADSM/2 support.
 The libraries have vendor-supplied management software that allows them to be 
employed as
very large random access disks.

   The results vary -
     One vendor's software allows the optical platter-sides to be merged into a 
single
logical drive.  Very nice for typical file service.  Their file system does 
not, however,
allocate space for empty files at the time they are created, it only creates 
directory
entries for them.  If the ADSM volumes (files) on the library are treated as 
'DISK' device
type, then ADSM attempts to scatter writes to the volumes, which would improve 
disk write
access.  With this vendor's optical software, however, writes to different 
volumes wind up on
the same platter.  Not bad for writes - terrible for reads.  For libraries 
using this
vendor's software, we have had to use 'FILE' type volumes to reduce the 
fragmentation created
by their handling of database-type writes.

     Another vendor's optical library management software allows any number of 
platter-sides
to be included in a single logical drive but, each platter-side becomes a 
distinct
sub-directory and data cannot span platter-sides.  For this software you have 
to create ADSM
volumes for each subdirectory (platter-side) but, you can use 'DISK' type 
volumes.  Again,
however, the way ADSM handles 'DISK' volume writes is not efficient with 
optical libraries -
since it tries to scatter writes to 'DISK' volumes within the same storage 
pool, so you
would still want to use 'FILE' type volumes.  This software does avoid the write
fragmentation experienced with the first vendor's software.


     Obviously, the way the second vendor handles volumes is preferable:
          -  Read performance is not impacted
          -  Each volume can be related to a specific platter-side


     Overall, optical library support I've seen so far has been pretty 
primitive - much
worse than I had anticipated.  Library file systems and management utilities 
are unreliable
and buggy, not anywhere near the reliability and resilience of HPFS.




------------------------------------------------
00:04:58    09/13/96
00:04:58    09/13/96
Paul C. Beck
Sr. Systems Engineer
Manassas Systems Integration Corp.
8575 King Carter Street
Manassas, VA 20110-4891

Voice/FAX:  703.257.0604
E-Mail:     beckpau AT ibm DOT net
WWW:        www.ManassasSystems.com
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