Compressed disk..

["Graap, Ken" <keg@xxxxxxxxxxxxxxxxxx>]

Hello everyone...

I've "discovered" something very interesting today. If you are using
"compressed disk" on your AS/400 you probably want to read on...

I have an S20 server. ASP2 in this server contains 10  8GB disks which have
been configured as compressed disk.

Yesterday I was experiencing problems with this S20 server. All disk I/O to
ASP2 was suspended and there was an SRC code on the control panel that
indicated all disk in ASP2 was full.

What is interesting about this is that the WRKDSKSTS command indicated that
the disk was only 58-65% full....

How can this be?

After a long conversation with IBM SupportLine I found out that the disk
measurement algorithm in OS/400 makes some assumptions. 

One of these assumptions is that if you configure a disk as a "compressed
disk" you will get at least a 2 to 1 compression on any data written to that
disk. Therefore it assumes that a physical 8GB drive is now a virtual 16GB
drive.

This was probably more of a marketing decision that a technical one. I can
hear the IBM marketing team now... "Lets show the disk as being twice as
big!"  "The customer will think that they got a 16GB drive for the cost of
an 8GB drive!!!"

Well, no matter what the operating system reports, the disk is still an 8GB
disk.

Our problem was that we were sending lots of data to this ASP via NFS, that
was already compressed. The disk IOP could not compress it any further,
therefore the assumption that it could be compressed at least 2 to 1 didn't
hold. We filled up our 10-8GB disk subsystem with this compressed data while
the WRKDSKSTS command only reported that we were around 60% full...

IBM is now aware of this problem. I'm waiting to hear from them if they are
going to do something about it.

In the mean time I have quite a challenge in how I'm going to manage my disk
space.  Especially since I never really know how much disk is actually being
used.


Kenneth

--
********************************
Kenneth E. Graap             
IBM Certified Specialist                 
AS/400  Professional                   
System  Administrator                  
NW Natural - Information Services        
System Services                         
Phone: 503 226 4211 X5537                 
FAX:   603 849 0591                  
keg@nwnatural.com               
********************************

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