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Single system imageFrom Wikipedia, the free
Indistributed computing, asingle system image(SSI) cluster is aclusterof
machines that appears to be one single system.The concept is often considered
synonymous with that of adistributed operating system,but a single image may be presented
for more limited purposes, justjob schedulingfor instance, which may be achieved by
means of an additional layer of software over conventionaloperating system
imagesrunning on eachnode.The interest in SSI clusters is based
on the perception that they may be simpler to use and administer than more
Different SSI systems may
provide a more or less complete illusion of a single system.
Different SSI systems may,
depending on their intended usage, provide some subset of these features.
Many SSI systems provide
process migration.Processes may start on onenodeand be moved to another node, possibly
forresource balancingor administrative reasons.As processes are moved from one node
to another, other associated resources (for exampleIPCresources)
may be moved with them.
Some SSI systems allowcheckpointingof running processes, allowing their
current state to be saved and reloaded at a later date.Checkpointing can be seen as related
to migration, as migrating a process from one node to another can be
implemented by first checkpointing the process, then restarting it on another
node. Alternatively checkpointing can be considered asmigration to disk.
Single process space
Some SSI systems provide
the illusion that all processes are running on the same machine - the process
management tools (e.g. "ps", "kill" onUnixlike systems) operate on all processes
in the cluster.
Most SSI systems provide a
single view of the file system. This may be achieved by a simpleNFSserver,
shared disk devices or even file replication.
The advantage of a single
root view is that processes may be run on any available node and access needed
files with no special precautions. If the cluster implements process migration
a single root view enables direct accesses to the files from the node where the
process is currently running.
Some SSI systems provide a
way of "breaking the illusion", having some node-specific files even
in a single root.HPTruClusterprovides a "context dependent
symbolic link" (CDSL) which points to different files depending on the
node that accesses it.HPVMSclusterprovides a search list logical name
with node specific files occluding cluster shared files where necessary. This
capability may be necessary to deal withheterogeneousclusters, where not all nodes have the
same configuration. In more complex configurations such as multiple nodes of
multiple architectures over multiple sites, several local disks may combine to
form the logical single root.
Single I/O space
Some SSI systems allow all
nodes to access the I/O devices (e.g. tapes, disks, serial lines and so on) of
other nodes. There may be some restrictions on the kinds of accesses allowed
(For exampleOpenSSIcan't mount disk devices from one node
on another node).
In most cases inter-node
IPC will be slower than IPC on the same machine, possibly drastically slower
for shared memory. Some SSI clusters include special hardware to reduce this
Cluster IP address
Some SSI systems provide a
"cluster address", a single address visible from outside the cluster
that can be used to contact the cluster as if it were one machine. This can be
used for load balancing inbound calls to the cluster, directing them to lightly
loaded nodes, or for redundancy, moving the cluster address from one machine to
another as nodes join or leave the cluster.