Sizing Guide Exchange Server 2003 - Fujitsu

White Paper Sizing Guide Exchange Server 2003 Version: 4.2, July 2006
Hard disks
A major influence on the performance is the speed of the hard disk. In addition to mean access time,
rotational speed in particular is an important parameter here. The faster the disk rotates, the more quickly the
data of a whole track can be transferred; but also the data density of the disk has an influence upon this. The
closer the data are together on the disk, i.e. the more data can be packed into one track, the more data can
be transferred per revolution and without repositioning the heads.
In the SCSI and SAS environment
only disks from the top of the
performance range are offered. In
this way, no hard disks are offered
with less than 10000 rpm
Type
2½" SATA
3½"
rpm
7200
7200
36 60
×
73
Capacity [GB]
80 100 146 160 250 300 500
×
× × × ×
(revolutions per minute) and a seek 2½" SAS 10000 × ×
time (positioning time) greater than 3½" SCSI 10000 × × × ×
6 ms. The table opposite shows the
currently available disk types. Hard
3½" 15000 × × ×
disks with even greater capacities are to be expected in the near future.
The rotational speed of the hard disk is directly reflected in the number of read/write
operations that a disk can process per time unit. If the number of I/O commands that
an application produces per second is known, it is possible to calculate the number
of hard disks required to prevent the occurrence of a bottleneck. In comparison with
5400 rpm
7200 rpm
IO/s
62
75
a hard disk with 10 krpm, a hard disk with 15 krpm shows - depending on the access 10000 rpm 120
pattern - an up to 40% higher performance, particularly in the case of random
accesses with small block sizes as occur with the Exchange database. For
15000 rpm 170
sequential accesses with large block sizes that occur in backup and restore processes, the advantage of
15 krpm is reduced to between 10% and 12%.
Moreover, the number of hard disks in an RAID array plays a major role. Thus, for example, eight 36 GB
disks in a RAID 1+0 are substantially faster than two 146 GB disks, although the result is the same effective
capacity. In other words, it is necessary to calculate between the number of available slots for hard disks, the
required disk capacity and ultimately the costs. From a performance point of view, the rule of more small
hard disks rather than less large ones is applicable.
If the Exchange Server 2003 is placed
under stress using the medium load
profile of LoadSim 2003, then 0.6 I/Os
per second and user will occur for the
Exchange database. The table
opposite shows the required number of
hard disks subject to the number of
users, disk rotational speed and RAID
level. It takes into consideration that
write accesses need two I/O
operations with a RAID 10 and up to
four I/O operations with a RAID 5. If
you also take the typical database access profile for Exchange with 2 /3 read and 1 /3 write accesses as the
basis, the I/O rate for a RAID 10 is calculated according to the formula
and the I/O rate for a RAID 5 according to the formula
# Users IO/s RAID 10 RAID 5
# IO # Disks # IO # Disks
10 krpm 15 krpm 10 krpm 15 krpm
50 30 40 2 2 60 3 3
100 60 80 2 2 120 3 3
500 300 400 4 4 600 5 4
1000 600 800 8 6 1200 10 8
2000 1200 1600 14 10 200 20 15
3000 1800 2400 20 16 3600 30 22
4000 2400 3200 28 20 4800 40 29
5000 3000 4000 34 24 6000 50 36
However, it should be noted that the actually required number depends on user behavior: a different user
profile can initiate a different I/O load.
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