HP StoreAll Storage - Manual (Page 9)

(Specifically, a segment need not be a complete, rooted directory tree). Segments can be any
size and different segments can be different sizes.
2. The location of files and directories within particular segments in the file space is independent
of their respective and relative locations in the namespace. For example, a directory (Dir1)
can be located on one segment, while the files contained in that directory (File1 and File2)
are resident on other segments. The selection of segments for placing files and directories is
done dynamically when the file/directory is created, as determined by an allocation policy.
The allocation policy is set by the system administrator in accordance with the anticipated
access patterns and specific criteria relevant to the installation (such as performance and
manageability). The allocation policy can be changed at any time, even when the file system
is mounted and in use. Files can be redistributed across segments using a rebalancing utility.
For example, rebalancing can be used when some segments are too full while other have free
capacity, or when files need to be distributed across new segments.
3. Segment servers are responsible for managing individual segments of the file system. Each
segment is assigned to one segment server and each server may own multiple segments, as
shown by the color coding in the diagram. Segment ownership can be migrated between
servers with direct access to the storage volume while the file system is mounted. For example,
Seg1 can be migrated between SS1 and SS2 but not to SS3 or SS4.
Additional servers can be added to the system dynamically to meet growing performance
needs, without adding more capacity, by distributing the ownership of existing segments for
proper load balancing and utilization of all servers. Conversely, additional capacity can be
added to the file system while in active use without adding more servers—ownership of the
new segments is distributed among existing servers. Servers can be configured with failover
protection, with other servers being designated as standby servers that automatically take
control of a server’s segments if a failure occurs.
4. Clients run the applications that use the file system. Clients can access the file system either
as a locally mounted cluster file system using the X9000 Client or using standard network
attached storage (NAS) protocols such as NFS and Common Internet File System (CIFS).
5. Use of the X9000 Client on a client system has some significant advantages over the NAS
approach—specifically, the X9000 Client driver is aware of the segmented architecture of
the file system and, based on the file/directory being accessed, can route requests directly to
the correct segment server, yielding balanced resource utilization and high performance.
However, the X9000 Client is available only for a limited range of operating systems.
6. NAS protocols such as NFS and CIFS offer the benefits of multi-platform support and low cost
of administration of client software, as the client drivers for these protocols are generally
available with the base operating system. When using NAS protocols, a client must mount
the file system from one (or more) of the segment servers. As shown in the diagram, all requests
are sent to the server from which the share is mounted, which then performs the required
routing.
7. Any segment server in the namespace can access any segment. There are three cases:
a.
Selected segment is owned by the segment server initiating the operation (for example,
SS1 accessing Seg1).
b. Selected segment is owned by another segment server but is directly accessible at the
block level by the segment server initiating the operation (for example, SS1 accessing
Seg3).
c. Selected segment is owned by another segment server and is not directly accessible by
the segment server initiating the operation (for example, SS1 accessing Seg5).
Each case is handled differently. The data paths are shown in heavy red broken lines in the
diagram:
a.
The segment server initiating the operation services the read or write request to the local
segment.
b. In this case, reads and writes take different routes:
File system operations 9