Editing OpenSSI (section)

===Single root===
OpenSSI provides a single root for the cluster - from any node the same files and directories are available.  OpenSSI uses several mechanisms to provide the single root – CFS (the OpenSSI Cluster File System), SAN cluster filesystems and parallel mounts of network file systems.

OpenSSI uses the context dependent symbolic link (CDSL) feature, inspired by HP's [[TruCluster]] system, to allow access to node-specific files in a manner transparent to non [[cluster-aware application]]s.  A CDSL may point to different files on each node in the cluster.

====CFS====
CFS, the OpenSSI Cluster File System provides transparent inter-node access to an underlying ''real'' file system on one node.

CFS is ''stacked'' on top of the real file system and co-ordinates access from different nodes using a ''token'' mechanism.  One node has physical access to the underlying file system and performs all read and write operations.  At any one time one node ''owns'' a token, representing a part of the underlying file, this implies that that part of the file is in the cache of the owning node.  If another node tries to access that part of the file the token is ''stolen'' and the cache contents are copied to the stealing node.  The OpenSSI CFS implementation is remarkably similar to that used by [[Hewlett-Packard|HP]] [[TruCluster]].<ref>
{{Citation
 |title = TruCluster Server Handbook
 |first1 = Scott
 |last1 = Fafrak
 |first2 = Jim A.
 |last2 = Lola
 |first3 = Brad
 |last3 = Nichols
 |first4 = Gregory
 |last4 = Yates
 |pages = 342–345
 |publisher = Digital Press
 |isbn = 1-55558-259-1
 |year = 2003
}}
</ref>

CFS is also used to co-ordinate access to shared memory segments.

CFS can be used in a fault tolerant system by using shared disk subsystems (dual ported [[SCSI]] or [[Storage area network|SAN]]), or by using [[DRBD]].  If the node that is currently directly accessing the file system crashes then the CFS mount ''fails over'' to the other node that is directly connected to the disk and the cluster now accesses the file system via that node.

====SAN clustered file systems====
OpenSSI can use [[Storage area network|SAN]] based [[clustered file system]]s for its root provided they provide a [[POSIX]] compatible file system interface. [[Lustre (file system)|Lustre]] and [[Global File System]] have been tested, too.

With a clustered file system, each node mounts the file system in parallel and access to the files goes directly from the node to the file system.

====NFS====
OpenSSI mounts [[Network File System (protocol)|NFS]] files systems in parallel on each node.  Every node accesses the NFS server directly.