Hello Ram,
if the amount of LUNs matters or not depends on your storage sub-system and on
the amount of parallel I/O requests on each LUN (basically what Jonathan meant
with queues).
There are storage sub-systems where the amount of LUNs really matter, e.g. IBM
XIV (Doc WP101452): "In general, to yield better performance with XIV it is
best to allocate fewer volumes with larger size. Using a minimum number of
volumes needed for data separation and keeping the volume sizes bigger will
allow XIV to better utilize cache algorithms such as pre-fetch and LRU. For
data, use a small number of large LUNs (typically 2 - 4/8) LUNs). Each LUN
should be between 500G - 2TB, depending on size of your database. Using a small
number of large LUNs takes better advantage of XIV’s aggressive caching
technology and simplifies storage management. For log files, use only one LUN
and match its size to the space required by the database configuration."
However if you have less LUNs you are putting more "pressure" on less disk
queues (queue depth) which may can result in higher application I/O wait times
(not service time!) but all of this can be measured very easily (e.g. iostat)
to cross-check your load and possible impact.
Best Regards
Stefan Koehler
Independent Oracle performance consultant and researcher
Website: http://www.soocs.de
Twitter: @OracleSK
Jonathan Lewis hat am 19. Oktober 2017 um 08:39 geschrieben:--
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 on behalf of Ram Raman
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.
