Volume Shadowing
for OpenVMS does not depend on specific hardware in order to operate.
All shadowing functions, with the exception of minicopy, can be
performed on Alpha and VAX computers using the OpenVMS operating
system. The minicopy operation can be performed only on an OpenVMS
Alpha system. However, an OpenVMS VAX system can be a member of
an OpenVMS Cluster system that uses this feature.
Volume shadowing requires a minimum of:
One CPU
One mass storage controller
One of the following kinds of disk drives:
Digital Storage Architecture (DSA)
Small Computer Systems Interface (SCSI)
Fibre Channel
The following sections generically describe hardware support.
See the HP Volume Shadowing for OpenVMS Software Product
Description (SPD 27.29.xx) for more information.
Memory Requirements Starting with OpenVMS Version 7.3, the following additional
memory is required to run Volume Shadowing for OpenVMS:
24 KB per node are
required on OpenVMS Alpha systems; 5 KB per node are required on
OpenVMS VAX systems for the default settings of the SHADOW_MAX_UNIT
system parameter. These requirements are in effect even if you do
not use Volume Shadowing for OpenVMS, unless you change the default
setting. If this memory is not available, the node will not boot.
4.5 KB per shadow set per node is required. This amount of memory is required before a write bitmap can
be created. If this memory is not available, the mount fails (that
is, the shadow set is not mounted on the node). The MOUNT command
that fails will issue the following message:
%MOUNT-F-INSFMEM, insufficient dynamic memory
For every gigabyte of storage of a shadow set member,
2.0 KB per node is required for the write bitmaps for each shadow
set mounted on a node. (Each shadow set can have up to six write
bitmaps.) When calculating memory requirements, note that a two-member
shadow set with 50 GB per member counts as 50 GB, not 100 GB. For example, for a shadow set with 200 GB of storage per member,
420 KB of memory is required for its write bitmaps for every node
in the cluster. If this memory is not available on the node where
the write bitmap request occurs, the write bitmap is not created. If the master write bitmap is created but sufficient memory
is not available on another node on which the shadow set is subsequently
mounted, a local write bitmap is not created. If the WBM_OPCOM_LVL system
parameter is set to 1 (the default) or 2, the following OPCOM message
is displayed:
Unable to allocate local bitmap - running in degraded mode.
Writes from nodes without local bitmaps are registered with
the node on which the shadow set was first mounted.
These memory requirements are cumulative. For example, a system
with 10 shadow sets mounted, with each shadow set consisting of
50-GB member disks, would require an additional 1,119 KB of memory.
The calculation follows:
24 KB per node (regardless
of whether you use volume shadowing)
45 KB (10 shadow sets x 4.5 KB per unit mounted
on the system)
1000 KB (50 x 2.0 KB (per GB of disk size) x 10
shadow sets
1069 KB total memory required
Supported Devices The
requirements for the physical disks that form a shadow set follow:
Starting with OpenVMS Alpha Version
7.3-2, different size devices can be used to form a shadow
set. This functionality is called dissimilar device shadowing (DDS).
To use DDS, all systems that have mounted a shadow set whose members
differ in size must be running OpenVMS Alpha Version 7.3-2. Prior to OpenVMS Alpha Version 7.3-2, Volume Shadowing
for OpenVMS required that all members of a shadow set be the same
size, that is, that each member have the exact same number of blocks.
The rapid advance of disk technology has made this requirement impractical.
The flexibility of using different size devices outweighs the space
that will be unused on the larger device. Operationally, shadowing dissimilar devices means that you
can add a larger disk device to an existing shadow set. The shadow
set retains the file system size of the original shadow set. After
adding a larger disk, if you remove a smaller disk, the geometry
(cylinders, tracks, and sections) of the shadow set changes to the
smallest remaining disk, but the logical volume size (that is, the
file system size) is not changed. All members of the shadow set must have
a MAXBLOCK size equal to or greater than the logical volume size
stored in the storage control block SCB$L_VOLSIZE of the shadow
set. All mounted members will have this value.When the smaller volume
is no longer needed, or if you need to increase the file system size
of the shadow set, you can use the dynamic volume expansion (DVE)
feature introduced in OpenVMS Alpha Version 7.3-2. Together,
the features of DDS and DVE enable you to continually grow a logical volume
without ever having to take it offline again. For more information
about DVE, see
Dynamic Volume Expansion. You can determine the block size for each disk with the SHOW
DEVICE /FULL command. The block size is displayed as Total
blocks nnnnnnnn.
Disks must be Files-11 On-Disk Structure Level 2
(ODS-2) or On-Disk Structure Level 5 (ODS-5) data disks. The Files-11
structure prepares a volume to receive and store data so that the
operating system can locate it easily. Volume shadowing accepts
I/O requests from users and applications through the Files-11 interface
and shadows data to the individual shadow set members.
Disks and controllers must be one of the following
types:
StorageWorks Fibre Channel
StorageWorks SCSI
MSCP (mass storage control protocol) conformant
Disk volumes cannot
have hardware write protection enabled. Hardware write protection
stops volume shadowing software from maintaining identical volumes.
SCSI disks that do not implement READL and WRITEL
have limited support because these disks do not provide for shadowing
data repair (disk bad block errors) features. Such disks can cause
members to be removed from the shadow set, if certain error conditions
arise that cannot be corrected. See
Using SDA to Obtain Information About Third-Party SCSI Devices for how to determine whether a SCSI disk
supports READL and WRITEL commands.