Dear Fellow Managers,
My deepest gratitude to :
"Mullin, Stephen (CAP, ITS, US)" <SMullin_at_gecits.GE.com>
Jon Wright <jwright_at_sloppy.spudboy.com>
Gyula Szokoly <szgyula_at_tarkus.pha.jhu.edu>
John Becker <John.Becker_at_digital.com>
George Guethlein <GGuethlein_at_GiantOfMaryland.com>
Alan Rollow <alan_at_nabeth.cxo.dec.com>
Mervyn Weis <Mervyn.Weis_at_digital.com>
Bill David <wdavid_at_accustaff.com>
Ronny Eliahu <ronny_eliahu_at_corp.disney.com>
Peter.Braack_at_degussa.de
I hope I haven't left anyone out.
Essentially my three questions were, with regard to configuring
a RAID 0+1 storage array, should I mirror a stripe set or should
I create a stripe set whose elements are mirrored disks. The actual
questions were:
1: Does the controller support both ways?
2: What are the issues one should consider in deciding which
way to use?
3: Is any way more robust or failure tolerant than the other?
3a: If So Which way is more so?
I shall give a summary of the responses to each question.
1: The HSZ40 controllers only seems to have software support for
configuring
Stripe sets whose members are mirrored disks. In addition several
responses stated that if I were to configure a mirror of a stripeset
I am more likely to suffer total failure because failure of one
member of either stripe set could cause complete loss of redundancy.
2: The first issue to consider was that of availability of the data.
It seems that a stripe set whose members are mirrored disks
is much more tolerant to failures.
Another issue is the performance issue. Though not an issue in
deciding
which method to use, there exists the option of doing things like
selecting a preferred path for/with dual-redundant controllers, and
allowing the controller to balance read requests among members of a
mirror set. This is capable of significantly improving the performance
of the
array.
3: Configuring a stripe set whose members are mirrored disks is
definitely the
way to go. It is much more forgiving and flexible. Additionally,
Striped
Mirrorsets allow faster recovery as there is only one disk to recover.
Additionally, several tips were offered which I shall quote in no
particular order.
* George Geuthlein pointed me to Chapter 8 of the RAIDbook which gave
some very good
reasoning and suggestions. I shall make a later post with the
information on how to
get that book. I am ordering one for myself.
* Some suggested I should upgrade to HSZ50 or HSZ70 controllers.
* Using the controller to create RAID 5 sets and then use LSM to do
mirroring may
be less costly. [Gyula Szokoly]
* It may be less costly to use a RAID 5 or RAID 3 set (But we REALLY
MUST have that
data online all the time).
* ALWAYS use dual-redundant controllers (controller pairs)
* Possibility: Use LSM to mirror across controller pairs
* ALWAYS be sure to allocate Spare Sets
* The syntax of the HSZ commands definitely prohibit creating mirrored
stripesets
* ALWAYS be sure to put different mirrorset members on separate channels
of the
controller. Prevents loss of the entire morrorset if the cable or the
channel
interface should fail.
* Use write-back caching to improve write performance. [CAVEAT: Check
the dates on the HSZ40
Cache batteries. There was an FCO on them I learned.]
Yours sincerely,
Robert Honore
robert_at_digi-data.com
> Take care of what IS. The APPEARANCES will take care of themselves.
> Money can't buy happiness, but the LACK of money can't buy ANYthing.
Statements appearing in this message and all components thereof
represent
the opinions of Robert Honore and do not necessarily represent those
of Digi-Data Systems, Limited.
Received on Fri Jan 23 1998 - 14:02:08 NZDT