ADSM-L

Re: [ADSM-L] High Mig Pct & Low Mig Pct = 100%

2009-04-15 13:58:51
Subject: Re: [ADSM-L] High Mig Pct & Low Mig Pct = 100%
From: Colin Dawson <colind AT US.IBM DOT COM>
To: ADSM-L AT VM.MARIST DOT EDU
Date: Wed, 15 Apr 2009 10:58:06 -0700
Howdy,

    LOWMIG can only be set as high as 99.  So, as a safety net you could
set LOWMIG=99 which would basically prevent migration from doing very much
in the event that it did even get started with the  HIGHMIG=100 setting...
After reviewing the code, while the HIGHMIG=100 will typically prevent
migration from running at all, if the pool occupancy ended up being exactly
100% utilitized, then that could also result in a case that migration could
start such that having the LOWMIG setting in place would further prevent
the data from migrating from the pool.  The thing to consider here is that
we may report the pool as 100% utilitized even though there may be some
fractional space not yet allocated or able to be allocated given the
transaction load seen on the server.  Because of this "rounding", typically
HIGHMIG=100 is sufficient to prevent migration from running.

    It is also worth noting that if migration did get started in the
HIGHMIG=100 case, it will likely overshoot the LOWMIG=99 value.  We check
the existing storage pool occupancy against the LOWMIG threshold while
migration is running periodically.  Depending upon how much data was
selected as a candidate for migration and the transaction grouping and
such, the work that could be done by migration in this case will likely
result in more data being migrated then may have been expected by the
LOWMIG threshold setting.

     Looking at the documentation and such, another thing to consider would
be to submit a market requirement (product enhancement request) to have
DEFINE STGPOOL and UPDATE STGPOOL allow for MIGPROCESS to be set to 0 as a
mechanism for explicitly disabling automatic storage pool migration.
-----------------------------------------------------
Colin Dawson
TSM Server Development
colind AT us.ibm DOT com


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  |Eric Thompson <ethompson AT ITLIFELINE DOT NET>                              
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  |ADSM-L AT VM.MARIST DOT EDU                                                  
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  |04/15/2009 10:18 AM                                                          
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  |Re: [ADSM-L] High Mig Pct & Low Mig Pct = 100%                               
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Thanks for the reply.

If I don't set the LOWMIG=100, then when the storage pool hits 100%,
won't it start a migration process that will continue until it hits the
LOWMIG value?

My thought was that if I set both to 100, then a migration would not
take place ever.  I would then depend on my secondary pool to accept any
unexpected "excess data".  (which shouldn't occur, since we size our
primary storage pools (disk) to handle all data, but occasionally we get
an unexpected influx that we want to overflow to tape temporarily until
it can be "looked at").

Thanks again.

Eric Thompson
Senior Storage Engineer
www.it-lifeline.com

-----Original Message-----
From: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU] On Behalf Of
Colin Dawson
Sent: Wednesday, April 15, 2009 10:07 AM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: [ADSM-L] High Mig Pct & Low Mig Pct = 100%

Howdy,

      We've seen many cases where folks set HIGHMIG=100.  I can't say
that
I've seen much where LOWMIG=100 is set or used in this fashion though...
Generally, HIGHMIG=100 is all that is needed to prevent migration from
trying to migrate the data from a given storage pool.

     The decision to overflow to the NEXT pool is done at the time that
the
server is trying to reserve space in a given pool.  If there is not
sufficient space available to reserve based on the incoming store
request,
it is at that point that the server will traverse to the NEXT pool and
attempt reservation/allocation in that pool.  The reservation and
allocation processing in this regard is pretty well independent of the
HIGHMIG setting for the storage pool.

-----------------------------------------------------
Colin Dawson
TSM Server Development
colind AT us.ibm DOT com


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  |[ADSM-L] High Mig Pct & Low Mig Pct = 100%
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I want to keep all data on the primary storage pool (disk), yet allow
for overflows to the secondary pool (tape) in the event more data is
backed up than expected.



Has anyone ever set both High & Low Mig Pct = 100% ?



If I do this, will the data still overflow to the secondary storage pool
during a backup if the primary storage pool is full (100%)?  Or will I
get the "out of storage space" message.



Thanks.  Eric