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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- Optical Libraries and ADSM, (continued)
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- Re: Optical Libraries and ADSM,
Paul C. Beck <=
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