Know more about HARD DISKS AND INTERNAL I/O CHANNEL
The controller of the disk subsystem must ultimately store all data on physical hard disks.Standard hard disks that range in size from 18 GB to 250 GB are currently (2003) usedfor this purpose. Since the maximum number hard disks that can be used is often limited,
the size of the hard disk used gives an indication of the maximum capacity of the overalldisk subsystem.When selecting the size of the internal physical hard disks it is necessary to weigh therequirements of maximum performance against those of the maximum capacity of theoverall system. With regard to performance it is often beneficial to use smaller hard disksat the expense of the maximum capacity: given the same capacity, if more hard disks areavailable in a disk subsystem, the data is distributed over several hard disks and thus theoverall load is spread over more arms and read/write heads and usually over more I/Ochannels (Figure 2.4). For most applications, medium-sized hard disks are sufficient. Onlyfor applications with extremely high performance requirements should smaller hard disks
the size of the hard disk used gives an indication of the maximum capacity of the overalldisk subsystem.When selecting the size of the internal physical hard disks it is necessary to weigh therequirements of maximum performance against those of the maximum capacity of theoverall system. With regard to performance it is often beneficial to use smaller hard disksat the expense of the maximum capacity: given the same capacity, if more hard disks areavailable in a disk subsystem, the data is distributed over several hard disks and thus theoverall load is spread over more arms and read/write heads and usually over more I/Ochannels (Figure 2.4). For most applications, medium-sized hard disks are sufficient. Onlyfor applications with extremely high performance requirements should smaller hard disks
be considered. However, consideration should be given to the fact that more modern,larger hard disks generally have shorter seek times and larger caches, so it is necessaryto carefully weigh up which hard disks will offer the highest performance for a certain
load profile in each individual case.Standard I/O techniques such as SCSI and Fibre Channel, to an increasing degree
SATA (Serial ATA) and sometimes also SSA (Serial Storage Architecture) are very oftenused for the internal I/O channels between connection ports and controller and betweencontroller and internal hard disks. Sometimes, however, proprietary – i.e. manufacturer-
specific – I/O technologies are used. Regardless of the I/O technology used, the I/Ochannels can be designed with built-in redundancy in order to increase the fault-toleranceof the disk subsystem. The following cases can be differentiated here:• ActiveIn active cabling the individual physical hard disks are only connected via one I/Ochannel (Figure 2.5, left). If this access path fails, then it is no longer possible toaccess the data.• Active/passiveIn active/passive cabling the individual hard disks are connected via two I/O channels
(Figure 2.5, right). In normal operation the controller communicates with the hard disksvia the first I/O channel and the second I/O channel is not used. In the event of thefailure of the first I/O channel, the disk subsystem switches from the first to the second
load profile in each individual case.Standard I/O techniques such as SCSI and Fibre Channel, to an increasing degree
SATA (Serial ATA) and sometimes also SSA (Serial Storage Architecture) are very oftenused for the internal I/O channels between connection ports and controller and betweencontroller and internal hard disks. Sometimes, however, proprietary – i.e. manufacturer-
specific – I/O technologies are used. Regardless of the I/O technology used, the I/Ochannels can be designed with built-in redundancy in order to increase the fault-toleranceof the disk subsystem. The following cases can be differentiated here:• ActiveIn active cabling the individual physical hard disks are only connected via one I/Ochannel (Figure 2.5, left). If this access path fails, then it is no longer possible toaccess the data.• Active/passiveIn active/passive cabling the individual hard disks are connected via two I/O channels
(Figure 2.5, right). In normal operation the controller communicates with the hard disksvia the first I/O channel and the second I/O channel is not used. In the event of thefailure of the first I/O channel, the disk subsystem switches from the first to the second
I/O channel.
Active/active (no load sharing)In this cabling method the controller uses both I/O channels in normal operation(Figure 2.6, left). The hard disks are divided into two groups: in normal operationthe first group is addressed via the first I/O channel and the second via the second I/Ochannel. If one I/O channel fails, both groups are addressed via the other I/O channel.Active/active (load sharing)In this approach all hard disks are addressed via both I/O channels in normal operation(Figure 2.6, right). The controller divides the load dynamically between the two I/Ochannels so that the available hardware can be optimally utilized. If one I/O channelfails, then the communication goes through the other channel only.Active cabling is the simplest and thus also the cheapest to realize but offers no protec-
on against failure. Active/passive cabling is the minimum needed to protect against fail-e, whereas active/active cabling with load sharing best utilizes the underlying hardware.
on against failure. Active/passive cabling is the minimum needed to protect against fail-e, whereas active/active cabling with load sharing best utilizes the underlying hardware.
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