V73 Features, System Management Features, Volume Shadowing for OpenVMS
*Conan The Librarian (sorry for the slow response - running on an old VAX)
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VMS Help V73 Features, System Management Features, Volume Shadowing for OpenVMS *Conan The Librarian (sorry for the slow response - running on an old VAX) |
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Volume Shadowing for OpenVMS introduces three new features, the
minicopy operation enabled by write bitmaps, new qualifiers for
disaster tolerant support for OpenVMS Cluster systems, and a new
/SHADOW qualifier to the INITIALIZE command. These features are
described in this section.
| 1 - Minicopy in Compaq Volume Shadowing (Alpha) |
This new minicopy feature of Compaq Volume Shadowing for OpenVMS
and its enabling technology, write bitmaps, are fully implemented
on OpenVMS Alpha systems. OpenVMS VAX nodes can write to shadow
sets that use this feature but they can neither create master
write bitmaps nor manage them with DCL commands. The minicopy
operation is a streamlined copy operation. Minicopy is designed
to be used in place of a copy operation when you return a shadow
set member to the shadow set. When a member has been removed from
a shadow set, a write bitmap tracks the changes that are made to
the shadow set in its absence, as shown in Application Writes to
a Write Bitmap.
When the member is returned to the shadow set, the write bitmap
is used to direct the minicopy operation, as shown in Member
Returned to the Shadow Set (Virtual Unit). While the minicopy
operation is taking place, the application continues to read and
write to the shadow set.
Thus, minicopy can significantly decrease the time it takes
to return the member to membership in the shadow set and can
significantly increase the availability of the shadow sets that
use this feature.
Typically, a shadow set member is removed from a shadow set to
back up the data on the disk. Before the introduction of the
minicopy feature, Compaq required that the virtual unit (the
shadow set) be dismounted to back up the data from one of the
members. This requirement has been removed, provided that the
guidelines for removing a shadow set member for backup purposes,
as documented in Volume Shadowing for OpenVMS, are followed.
For more information about this new feature, including additional
memory requirements for this version of Compaq Volume Shadowing
for OpenVMS, refer to Volume Shadowing for OpenVMS.
| 2 - Multiple-Site OpenVMS Cluster Systems |
OpenVMS Version 7.3 introduces new command qualifiers for the
DCL commands DISMOUNT and SET for use with Volume Shadowing for
OpenVMS. These new command qualifiers provide disaster tolerant
support for multiple-site OpenVMS Cluster systems. Designed
primarily for multiple-site clusters that use Fibre Channel for
a site-to-site storage interconnect, they can be used in other
configurations as well. For more information about using these
new qualifiers in a multiple-site OpenVMS Cluster system, see the
white paper Using Fibre Channel in a Disaster-Tolerant OpenVMS
Cluster System, which is posted on the OpenVMS Fibre Channel web
site at:
http://www.openvms.compaq.com/openvms/fibre/
The new command qualifiers are described in this section. Using
DISMOUNT and SET Qualifiers describes how to use these new
qualifiers.
DISMOUNT/FORCE_REMOVAL ddcu:
One new qualifier to the DISMOUNT command, DISMOUNT/FORCE_REMOVAL
ddcu:, is provided. If connectivity to a device has been lost and
the shadow set is in mount verification, /FORCE_REMOVAL ddcu: can
be used to immediately expell a named shadow set member (ddcu:)
from the shadow set. If you omit this qualifier, the device is
not dismounted until mount verification completes. Note that this
qualifier cannot be used in conjunction with the /POLICY=MINICOPY
(=OPTIONAL) qualifier.
The device specified must be a member of a shadow set that is
mounted on the node where the command is issued.
SET DEVICE
The following new qualifiers to the SET DEVICE command have
been created for managing shadow set members located at multiple
sites:
o /FORCE_REMOVAL ddcu:
If connectivity to a device has been lost and the shadow set
is in mount verification, this qualifier causes the member to
be expelled from the shadow set immediately.
If the shadow set is not currently in mount verification, no
immediate action is taken. If connectivity to a device has
been lost but the shadow set is not in mount verification,
this qualifier lets you flag the member to be expelled from
the shadow set, as soon as it does enter mount verification.
The device specified must be a member of a shadow set that is
mounted on the node where the command is issued.
o /MEMBER_TIMEOUT=xxxxxx ddcu:
Specifies the timeout value to be used for a member of a
shadow set.
The value supplied by this qualifier overrides the SYSGEN
parameter SHADOW_MBR_TMO for this specific device. Each member
of a shadow set can be assigned a different MEMBER_TIMEOUT
value.
The valid range for xxxxxx is 1 to 16,777,215 seconds.
The device specified must be a member of a shadow set that is
mounted on the node where the command is issued.
o /MVTIMEOUT=yyyyyy DSAnnnn:
Specifies the mount verification timeout value to be used for
this shadow set, specified by its virtual unit name, DSAnnnn.
The value supplied by this qualifier overrides the SYSGEN
parameter MVTIMEOUT for this specific shadow set.
The valid range for yyyyyy is 1 to 16,777,215 seconds.
The device specified must be a shadow set that is mounted on
the node where the command is issued.
o /READ_COST=zzz ddcu:
The valid range for zzz is 1 to 4,294,967,295 units.
The device specified must be a member of a shadow set that is
mounted on the node where the command is issued.
This qualifier allows you to modify the default "cost"
assigned to each member of a shadow set, so that reads are
biased or prioritized toward one member versus another.
The shadowing driver assigns default READ_COST values to
shadow set members when each member is initially mounted.
The default value depends on the device type, and its
configuration relative to the system mounting it. There are
default values for a DECRAM device; a directly connected
device in the same physical location; a directly connected
device in a remote location; a DECram served device; and a
default value for other served devices.
The value supplied by this qualifier overrides the default
assignment. The shadowing driver adds the value of the current
queue depth of the shadow set member to the READ_COST value
and then reads from the member with the lowest value.
Different systems in the cluster can assign different costs to
each shadow set member.
If the /SITE command qualifier has been specified, the
shadowing driver will take site values into account when it
assigns default READ_COST values. Note that in order for the
shadowing software to determine if a device is in the category
of "directly connected device in a remote location," the /SITE
command qualifier must have been applied to both the shadow
set and to the individual device.
Reads requested for a shadow set from a system at Site 1 are
performed from a shadow set member that is also at Site 1.
Reads requested for the same shadow set from Site 2 can read
from the member located at Site 2.
o /READ_COST=y DSAnnnn
The valid range for y is any non-zero number. The value
supplied has no meaning in itself. The purpose of this
qualifier is to switch the read cost setting for all shadow
set members back to the default read cost settings established
automatically by the shadowing software. DSAnnnn must be a
shadow set that is mounted on the node from which this command
is issued.
o /SITE=(nnn, logical_name) (ddcu: DSAnnnn:)
This qualifier indicates to the shadowing driver the site
location of the shadow set member or of the shadow set
(represented by its virtual unit name). Prior to using
this qualifier, you can define the site location in the
SYLOGICALS.COM command procedure to simplify its use.
The valid range for nnn is 1 through 255.
The following example shows the site locations defined,
followed by the use of the /SITE qualifier:
$ DEFINE/SYSTEM/EXEC ZKO 1
$ DEFINE/SYSTEM/EXEC LKG 2
$!
$! At the ZKO site ...
$ MOUNT/SYSTEM DSA0/SHAD=($1$DGA0:,$1$DGA1:) TEST
$ SET DEVICE/SITE=ZKO DSA0:
$!
$! At the LKG site ...
$ MOUNT/SYSTEM DSA0/SHAD=($1$DGA0,$1$DGA1) TEST
$ SET DEVICE/SITE=LKG DSA0:
$!
$! At both sites, the following would be used:
$ SET DEVICE/SITE=ZKO $1$DGA0:
$ SET DEVICE/SITE=LKG $1$DGA1:
o /COPY_SOURCE (ddcu:,DSAnnnn:)
Controls whether one or both source members of a shadow
set are used as the source for read data during full copy
operations, when a third member is added to the shadow
set. This only affects copy operations that do not use DCD
operations.
HSG80 controllers have a read-ahead cache, which significantly
improves single-disk read performance. Copy operations
normally alternate reads between the two source members, which
effectively nullifies the benefits of the read-ahead cache.
This qualifier lets you force all reads from a single source
member for a copy operation.
If the shadow set is specified, then all reads for full copy
operations will be performed from whichever disk is the
current "master" member, regardless of physical location of
the disk.
If a member of the shadow set is specified, then that member
will be used as the source of all copy operations. This allows
you to choose a local source member, rather than a remote
master member.
o /ABORT_VIRTUAL_UNIT DSAnnnn:
To use this qualifier, the shadow set must be in mount
verification. When you specify this qualifier, the shadow
set aborts mount verification immediately on the node from
which the qualifier is issued. This qualifier is intended to
be used when it is known that the unit cannot be recovered.
Note that after this command completes, the shadow set must
still be dismounted. Use the following command to dismount the
shadow set:
DISMOUNT/ABORT DSAnnnn
2.1 - Using DISMOUNT and SET Qualifiers
The diagram in this section depicts a typical multiple-site
cluster using Fibre Channel. It is used to illustrate the steps
which must be taken to manually recover one site when the site-
to-site storage interconnect fails. Note that with current Fibre
Channel support, neither site can use the MSCP server to regain a
path to the DGA devices.
To prevent the shadowing driver from automatically recovering
shadow sets from connection-related failures, three steps must be
taken prior to any failure:
1. Every device that is a member of a multiple-site shadow set
must have its member_timeout setting raised to a high value,
using the following command:
$ SET DEVICE /MEMBER_TIMEOUT= x ddcu:
This command will override the SHADOW_MBR_TMO value, which
would normally be used for a shadow set member. A value for x
of 259200 would be a seventy-two hour wait time.
2. Every shadow set that spans multiple sites must have its mount
verification timeout setting raised to a very high value,
higher than the MEMBER_TIMEOUT settings for each member of the
shadow set.
Use the following command to increase the mount verification
timeout setting for the shadow set:
$ SET DEVICE /MVTIMEOUT = y DSAnnnn
The y value of this command should always be greater than the
x value of the $ SET DEVICE/MEMBER_TIMEOUT= x ddcu:.
The $ SET DEVICE /MVTIMEOUT = y command will override the
MVTIMEOUT value, which would normally be used for the shadow
set. A value for y of 262800 would be a seventy-three hour
wait.
3. Every shadow set and every shadow set member must have a site
qualifier. As already noted, a site qualifier will ensure that
the read cost is correctly set. The other critical factor is
three-member shadow sets. When they are being used, the site
qualifier will ensure that the master member of the shadow set
will be properly maintained.
In the following diagram, shadow set DSA42 is made up of
$1$DGA1000 and $1$DGA2000
<><><><><><><><><><><> LAN <><><><><><><><><><><>
Site A Site B
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F.C. SWITCH <><><><> XYZZY <><><><> F.C. SWITCH
| |
HSG80 <><> HSG80 HSG80 <><> HSG80
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$1$DGA1000 --------- DSA42 --------- $1$DGA2000
This diagram illustrates that systems at Site A or Site B have
direct access to all devices at both sites via Fibre Channel
connections. XYZZY is a theoretical point between the two sites.
If the Fibre Channel connection were to break at this point,
each site could access different "local" members of DSA42 without
error. For the purpose of this example, Site A will be the sole
site chosen to retain access to the shadow set.
The following actions must be taken to recover the shadow set at
Site A.
On Site A:
$ DISMOUNT /FORCE_REMOVAL= $1$DGA2000:
Once the command has completed, the shadow set will be available
for use only at site A.
On Site B:
$ SET DEVICE /ABORT_VIRTUAL_UNIT DSA42:
Once the command completes, the shadow set status will be
MntVerifyTimeout.
Next, issue the following command to free up the shadow set:
$ DISMOUNT/ABORT DSA42:
These steps must be taken for all affected multiple-site shadow
sets.
| 3 - Using INITIALIZE With SHADOW and ERASE Qualifiers |
The new /SHADOW qualifier to the DCL INITIALIZE command is
available. The use of the INITIALIZE /SHADOW command to
initialize multiple members of a future shadow set eliminates
the requirement for a full copy operation when you later create a
shadow set.
Compaq strongly recommends that you also specify the /ERASE
qualifier with the INITIALIZE/SHADOW command when initializing
multiple members of a future shadow set. Whereas the /SHADOW
qualifier eliminates the need for a full copy operation when
you later create a shadow set, the /ERASE qualifier reduces the
amount of time a full merge will take.
If you omit the /ERASE qualifier, and a merge operation of the
shadow set is subsequently required (because a system on which
the shadow set is mounted fails), the resulting merge operation
will take much longer to complete.
The INITIALIZE command with the /SHADOW and /ERASE qualifiers
performs the following operations:
o Formats up to six devices with one command, so that any three
can be subsequently mounted together as members of a new host-
based shadow set.
o Writes a label on each volume.
o Deletes all information from the devices except for the system
files containing identical file structure information. All
former contents of the disks are lost.
You can then mount up to three of the devices that you have
initialized in this way as members of a new host-based shadow
set.
For more information, refer to Volume Shadowing for OpenVMS.
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