Also to be remembered is the bitmap which tracks the blocks which are in use
grows along with the disk size. What O/S ( and db) vendors won't tell you is
how many concurrent changes can happen in the bitmap. This is capped to the
level they can test. The number has gone up over the years, but it is still
finite. You are more likely to hit that number when you have huge LUNS. It is
the same as having many tablespaces or just dumping everything into one
tablespace. I don't think any of us would create one tablespace only.
Liz
Elizabeth Reen
CPB Database Group Manager
718.248.9930 (Office)
Service Now Group: CPB-ORACLE-DB-SUPPORT
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx] On ;
Behalf Of Jonathan Lewis
Sent: Thursday, October 19, 2017 2:39 AM
To: ORACLE-L; veeeraman@xxxxxxxxx
Subject: Re: SSDs and LUNs
At some layer between Oracle and the silicon the various software components
will have some queues. If there is a layer at which you have a single queue to
each LUN you will have an I/O bottleneck when you've got lots of Oracle
processes trying to read from just 2 (or 4) LUNs.
I'm not an expert with stuff that far away from the Oracle software but I would
be a little surprised if you got bad performance because you were configured as
40 LUNs, while I have seen bad performance from a system where the solid state
SAN had been configured as just 2 LUNs (one for data, one for redo).
Regards
Jonathan Lewis
________________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Ram Raman <veeeraman@xxxxxxxxx>
Sent: 19 October 2017 06:49:23
To: ORACLE-L
Subject: SSDs and LUNs
We are moving one of the systems to vm. The consultants who have been hired to
do the implementation are recommending that we create just 2 or 4 'LUNS' for
data diskgroup for the db that is 3Tb in size which exhibits hybrid IO. They
are promising it is best rather than having 30 or 40 LUNs since the new disks
will all be SSDs.They are claiming that it will perform better than having 40
'LUNs'. I still have the 'old way of thinking' when it comes to IO. Can
someone confirm one way or other, or point to any paper. thanks.
Ram.
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