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You don’t mention the capacity of the disks you are looking
at. Here is something you might want to consider. I’ve seen a few performance posts
on using different hardware technologies to gain improvements. Most of those
comments are on raid, interface and rotation speed. One area that
doesn’t seem to have been mentioned is to run your
disks empty. One of the key roadblocks in disk
performance is the time for the disk heads to seek, settle and find the start
of the data. Another is the time to transfer from disk to interface. Everyone
may instinctively know this but its often ignored. Hard disks are CRV ( constant
rotational velocity) = they spin at the same speed all the time Hard disk drives use a
technology called ZBR = Zone Bit Recording = a lot more data on the
outside tracks than the inner ones. Hard disk fill up from outside track
to inside track generally unless you’ve done some weird
partitioning. On the outside of the disk you get
a lot more data per seek than on the inside. Double whammy you get it faster. Performance can vary more
than 100% between the outer and inner tracks of the disk. So running
a slower disk twice as big may give you more benefit than running a small capacity
15K disk full. The slower disks are also generally more reliable and mostly
much cheaper. The other issue for full disks
especially with lots of random small transactions is the heads are seeking and
settling across the whole disk but typically with most of those
seeks being on the latest transactions which are placed nicely towards the
middle of the disk. I know of a major bank that has a rule
of thumb 25% of the disk partioned as a target maximum for high
performance disk systems in a key application. They also only pay for used capacity
from their disk vendor. This is not very green as you need
to buy more disks for the same amount of data and its liable to upset your
purchasing department who won’t understand why you don’t want to
fill your disks up. Mike |
