A brief history of database disk storage
Oracle Database Tips by Donald BurlesonOctober 28, 2015
The duties of any database administrator
include responsibility for the management, control and safety of
mission-critical information, and a primary concern of the Oracle DBA is
managing their computer disk storage.
elegantly described the important of knowing history:
"The farther you can
look into the past, the farther that you can see into the future".
With this sage wisdom in-mind, lets
review the history of this all-important component of database
tuning. Also see my important notes on
tuning Oracle full-scan operations.
In the 1970's, punched cards were the prominent data
storage device, and the ubiquitous cards were even used as income tax refund checks.
People even made cute Christmas trees from punched cards
in the "Data Processing" department.
College-aged kids have no idea what the term "Do not fold, spindle
or mutilate" means and they missed-out on the fun of dropping your
card deck on the floor and using giant sorting machines to
re-sequence them (assuming you had the sequence numbers in all of
More than fifty years ago, IBM introduced
RAMAC (random access
accounting & control), the first in a long line of cumbersome and expensive
database storage architectures. Disk platter storage continued to evolve
through the 1990's, when engineers hit the physical limits of mechanical devices
and RAM was introduced as a front-end cache. In the early 21st century we
see RAM-SAN replacing the antiquated mechanical platters.
We also see that old habits die hard,
especially with regard to database technology. In database management
software, IBM's flagship IMS database still has a significant market share after
40 years on-the-job, IBM's mainframe DB2 database still stores most of the
world's data, and Oracle, the world's most flexible and robust DBMS, will soon
celebrate their 20th birthday.
Lets take a quick history trip to understand
the exact nature of disk storage within large database management systems.
Also see my related notes on the
future of database management oracle database management.
The age of spinning platters
One of the first commercial uses of computer
storage involved paper tape and punched cards, and these archaic data storage
methods were superseded by Direct Access Storage Devices, DASD (pronounced "daz-dee").
The most popular disk of the 1980's was the refrigerator-sized 3380 disks, which
contained only 1.2 gig of storage at the astronomical cost of over $200,000.
In today's 2015 dollars, disk in the 1980's costs more than $4,000.00 per
Today, you can buy 100
GB disks for $200, and 100 GB of RAM Disk (solid-state disk) for
A DBA in 1980's spent a great
deal of time managing their DASD spindles, huge mechanical devices with these
giant rotating platters of magnetic-coated media. RAID (and the whole concept
of "inexpensive" disk) was a decade away, and the DBA of the 1980's fought to
make every byte count, while still delivering acceptable performance to their
In order to reduce the latency of access, The
1980's DBA would place high-use datafile near the middle absolute track of the
device, and data placement was always a pain because of the huge size of the
disks and the high expense. See my book
Oracle Disk I/O
Tuning for details:
So, with the technology constantly re-vamping,
where have we come in the past half-century of disks? I think that it has
gotten worse, not better. Sure, disks are incredibly cheap compared to their
ancestors, but they still have shortcomings, and disks are getting worse, not
Plague of Large Oracle Disks
Placing too much data on a single disk
spindle can impose enqueues on any disk because the mechanical device can only
locate to a single cylinder at a time. On busy Oracle
databases on a single disk spindle, the disk can shake like an
out-of-balance washing machine as competing tasks enqueue for data
Originally, RAID was an acronym for INEXPENSIVE
DISKS, and Oracle professionals enjoyed the ability of spreading
their Oracle data across multiple disks and spreading the load.
Today's large disk arrays commonly utilize asynchronous write and
multi-gigabyte RAM buffers to minimize this latency, but it still is
a major concern for thousands of Oracle shops.
This problem of "single channel access"
(a mechanical bottleneck caused by the read-write heads of the disk
imposes a bottleneck on Oracle disk devices, and the large disks
(over 144 gigabytes) often perform more slowly for high concurrent
access than their smaller predecessors.
Disk enqueues can occur when the disk is unable to quickly service concurrent
requests. Super-large disks can be problematic, and the most popular
Oracle data files can be placed on the middle absolute track of the device to
minimize read-write head movement.
Oracle's standard SAME (Stripe and Mirror
Everywhere, RAID 10) is largely useless for load balancing if
the whole database resides on just a few physical disks.
Seek delay (movement of the read-write
heads) composes over 80% of disk access latency, and high
concurrent requests against large devices make them very slow.
As manufacturing costs continue to fall, disk
vendors are offering larger and larger disks and this is imposing
some serious large disk
performance issues with seek delay is 1.5 times slower:
"In the ?good old days? when 9G disks were
big, we didn't have this problem. Really, this
problem is new since then. Back then, if we
wanted 200G of storage RAID1, we needed about 45
of those disks.
Controllers could only handle 7 of them, you see
(the 8th device on the bus was the controller
itself) and that meant we had proportionally
lots more access roads, and lots more loading
docks per square foot of warehouse space than
you typically have today. . .
- The seek time is
- The bandwidth performance is maximum
only 10 times better
- Your controller is no more than 32
The disk, however is about 83
As a result of this trend, many Oracle
professionals experience external I/O waits and they see that the
top-5 waits events (from a STATSPACK or AWR Report) show "db file
sequential reads" and "db file scattered reads" as a main system
The disk vendors say that
is not dead, and that they will continue to get smaller and
"Drives with mere hundreds of gigabytes will be small enough
to wear as jewelry.
"You'll have with you every album and tune
you've ever bought, every picture you've ever taken, every tax
Reducing physical disk reads
see the following trends for Oracle disk in this decade:
costs fall 10x every year.
Magnetic-coated spinning platters are obsolete and cannot go
replaces disk by 2015.
Note the change to Moore's Law for disks show the limitations of the
spinning platter technology (figure 3).
Moore's Law for disk
Platters can only spin so fast without becoming aerodynamic and the
disk vendors were hard-pressed to keep their technology improving in
speed. Their solution was to add a RAM
front-end to their disk arrays and sophisticated asynchronous
read-write software to provide the illusion of faster hardware
In the 21st
century, the Oracle professional must take
action to relieve disk I/O bottlenecks. There are several
solutions to this issue:
Use large data buffer caches - The
majority of the Oracle 10g benchmarks (
www.tpc.org ) use 64-bit Oracle
with a db_cache_size over 50 gigabytes. Other large shops
segregate I/O into multiple data buffers by using Oracle
Get higher bandwidth storage
Oracle shops purchase the more-expensive smaller devices or disk
with fixed read-write heads (Winchester technology). Other
embrace SSD arrays which have unprecedented bandwidth for high
concurrent access since
Oracle SSD clobbers disk access speed.
It's clear that solid-state devices are making huge headway, and
several books on
SSD tuning and stepping-up to the new technology. With
prices falling rapidly, I expect that most database will be
solid-state in the next few years and that disk will become the "new
tape", offline tertiary storage for backups.
Disk history References
My other notes on Oracle disks include: