14.6.2.1 Native vs. Virtual Performance
Figure 1 shows a comparison of measured bandwidth using virtual SCSI and local attached
DASD for reads with varying block sizes of operations. The difference in the reads between
virtual I/O and native I/O in these tests is attributable to the increased latency using virtual I/O.
The difference in writes is caused by misalignment, which causes a read for every write. A write
alignment change is planned for a future IBM i operating system release which will make virtual
and native writes similar in speed.
Native vs. Virtual
0
20
40
60
80
100
Sm
a
ll
T
rans
ac
ti
ons
(4
k
-16k
)
M
e
dium
T
rans
ac
ti
ons
(32k
-64k
)
Lar
ge
T
rans
ac
ti
ons
(128
k
+
)
MB
/s
Native Read
Virtual Read
Native Write
Virtual Write
Figure 1 - The figure above shows a comparison of native vs virtual. This experiment shows that virtual write performance is
significantly less then native. Read performance performs similar or better then native depending on the read-cache
performance.
14.6.2.2 Virtual SCSI Bandwidth-Multiple Network Storage Spaces
Figure 2 shows a comparison of measured bandwidth while scaling network storage spaces with
varying block sizes of operations. The difference in the scaling of these tests is attributable to the
performance gain, which can be achieved by adding multiple network storage spaces. This
experiment shows that in order to achieve better performance from the hard disk, multiple
network storage spaces can be used.
15 Disk 5GB Sequential Read
0
20
40
60
80
100
120
140
160
180
200
4
8
16
32
64
128
256
I/ O Block Size
MB
p
s
1 NWSS
2 NWSS
4 NWSS
8 NWSS
15 Disk 5GB Sequential Write
0
10
20
30
40
50
4
8
16
32
64
128
256
I/O Block Size
MB
p
s
1 NWSS
2 NWSS
4 NWSS
8 NWSS
Figure 2- The figures above show performance while scaling network storage spaces. This experiment shows that adding
NWSS increases the throughput for read/write performance. The best performance is achieved by using 8 NWSS.
IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
©
Copyright IBM Corp. 2008
Chapter 14 DASD Performance
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