RE: ASM Record Deletions

  • From: "Kevin Closson" <kevinc@xxxxxxxxxxxxx>
  • To: <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 13 Jun 2006 16:32:45 -0700

 
         
        >>>>If I have 10 millions records in a table and if I drop the
table or delete 1 million records and if ASM_POWER_LIMIT to > 0, Is it
going to rebalance the Disk space among all Disks used in the Diskgroup.
This table is currently having 50G space.


Folks, absorb this one:

        ASM is not aware of the contents of blocks. 

ASM is a raw disk space manager. If you add or drop a disk, you
trigger a rebalancing act. If you drop a disk, say a lot of prayers
you are helping mankind by performing oracle QA.

If you add less than probably 100% disk capacity and endure a total
rebalance, the odds are actually pretty slim that you will get a 
performance increase. "Oh boy, he's making that up", they sigh. Two
words, test it.

ASM is not QoS. Not by any stratch of the imagination. Of course,
blind ASM fanatics will tell you you don't need QoS of any sort--since
ASM doesn't have it. 

Consider the fact that the majority of OLTP accesses hit the 
minority of the database blocks (usually on the order of 80% hit
some 20% of the data) ...and that minority of blocks 
moves.  If you have, say, 50 disks and you add 10 and go through a
rebalance, will the hot blocks get extra representation on the
new spindles? No. Should they? Yes. Who is implementing stuff like that?
Not Oracle. Guess who does?

QoS is the art of transparently promoting or demoting blocks of disk
to different levels of the storage hierarchy. Since IT shops have
generally
given up on actually caring about such elegance, you just don't get a 
lot of water-cooler talk about such. And, after all, if ASM doesn't do 
it, it must not be needed. Conversely, if ASM does do something, it must
be right.  

Ok, off to add 20 disks to my 200 disk diskgroup. I'll start with 
eleventeen Spaten Pils and count my way down until it is done (if it
ever finishes). Then I'll be properly anesthetized so that when I
measure the performance delta I wont be upset to find I got squat
out of all that monkey business. I'll then slumber dreaming of
ASM-ites enduring ATONTRI delights.
                                





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