RE: Disk space and performance

["Richard Jackson" <richardjackson@xxxxxxxxxxxxxxxxxx>]

It depends.

The 80 percent rule was created when DASD was 3370 devices on system 38.  It
still applied during the early years of AS/400 when disk subsystems used
drives between 500 megabytes and 2 gigabytes.

When the drives are that small, individual pieces of free space on each
drive are very small - smaller than the typical allocation sizes.  Thus when
database allocates a new extent of 4 megabytes, there may not be one chunk
of 4 megabytes of free space available.  When that happens, two or more
database extents are created.  Performance expectations are set based on
"average use" and the 4 megabyte extent size.  When there are more extents,
"average use" will require more disk arm seeks  for the same file.  Since a
disk seek takes from 3 to 50 milliseconds and tens of millions of them are
done each day, the numbers add up.

Notice that once a database allocation is made in two units, it stays in two
units and in that physical place forever - or until you rearrange disk ASPs,
delete the file and reclaim the space, clear or remove the member and
reclaim the space, or run disk balance.

The 80 percent rule applies more when the individual drives are small or
when the total disk space on the system is relatively small.  "Small" is
relative to the physical disk IO traffic that they have to support.
Physical disk IO depends on the applications, physical memory size, pool
sizes, purge settings (prior to V4R5 and V4R5), and activity levels.  As the
total system size increases, 80 percent of one terabyte is 800 gigabytes.
That means that the free space equals 200 gigabytes.  When the total free
space equals 200 gigabytes, there is a higher probability that you can find
a 4 megabyte free space on each drive.  If the size of each disk arm is
larger, there is also a greater probability that you can find a single free
space unit to allocate.  However, at this time, I believe that the 8gb 10k
RPM arms are the largest disk arms that are practical for transaction
processing.  I think that the 17 GB arms have their place but that place is
for online archives, not transaction data.

In my opinion, the 80 percent rule works well for systems using 4gb arms or
smaller and total disk space under about 80 gb.  Above either of those
ceilings, I think that you have to look at more details to predict when
system performance will be significantly affected.

The 80 gb number is pretty arbitrary - mostly a guess based on experience.
When you work it out, an 80 gb system that is 80 percent loaded has 16 gb of
free space.  If 4 gb disk arms are used - and no RAID or mirroring - that
means that each disk arm has about 800 megabytes of free space.  It seems
pretty likely that there is at least one four-megabyte chunk of free space
on that drive.  However, when the drive is 90 percent full, there is only
400 megabytes of free space and it seems pretty likely that there may _not_
be one four-megabyte chunk available.

Analyzing the same setup only with RAID-5, an 80 gigabyte system (organized
into 2 sets of 10 arms each) dedicates 8 gb (parity on 8 arms in each set of
10, one arm in each set is lot to parity) to store parity data.  The system
has 72 gb total capacity.  80 percent of 72 gb is 57.6 gb so free space
equals 14.4 gb spread over 20 arms equals 720 megabytes free space available
per arm.  This analysis is ignoring the fact that 2 arms in each set store
no parity data.  So instead of allocating 1 part in 200 (the "no protection"
case), in the RAID-5 case we are allocating 1 part in 180 - still pretty
good odds.  When the RAID-5 drives are 90 percent full, our odds are 1 part
in 90.

Different arrangements of RAID or mirroring will change the numbers and that
is why I suggest that more details are needed when the drive sizes or total
system sizes increase.

At this time, I do not know how to determine the size and arrangement of
free space on AS/400 disk drives so I do not know how to predict the effect
or limits for a particular installation.  I am not aware of anyone who knows
how to reliably do this.

Richard Jackson
mailto:richardjackson@richardjackson.net
http://www.richardjacksonltd.com
Voice: 1 (303) 808-8058
Fax:   1 (303) 663-4325

-|-----Original Message-----
-|From: owner-midrange-l@midrange.com
-|[mailto:owner-midrange-l@midrange.com]On Behalf Of Quazy
-|Sent: Wednesday, October 25, 2000 7:36 AM
-|To: midrange-l@midrange.com
-|Subject: Disk space and performance
-|
-|
-|I was wondering if this still holds true today. My boss says that
-|when you
-|let the as400 use up more than 80% of you disk performance will degrade.
-|
-|Thanks Chris.
-|
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