You can use the command-line interface (CLI) to create
a managed disk (MDisk) group.
Attention: If you add an MDisk to an MDisk group
as an MDisk, any data on the MDisk is lost. If you want to keep the
data on an MDisk (for example, because you want to import storage
that was previously not managed by SAN Volume Controller),
you must create image mode virtual disks (VDisks) instead.
Assume
that the cluster has been set up and that a back-end controller has
been configured to present new storage to the SAN Volume Controller.
If
you are using a SAN Volume Controller solid-state drive (SSD) managed
disk, ensure that you are familiar with the SSD configuration rules.
This
task assumes that you have already launched the SAN Volume Controller Console.
Consider
the following factors as you decide how many MDisk groups to create:
- A VDisk can only be created using the storage from one MDisk group.
Therefore, if you create small MDisk groups, you might lose the benefits
that are provided by virtualization, namely more efficient management
of free space and a more evenly distributed workload for better performance.
- If any MDisk in an MDisk group goes offline, all the VDisks in
the MDisk group go offline. Therefore you might want to consider using
different MDisk groups for different back-end controllers or for different
applications.
- If you anticipate regularly adding and removing back-end controllers
or storage, this task is made simpler by grouping all the MDisks that
are presented by a back-end controller into one MDisk group.
- All the MDisks in an MDisk group should have similar levels of
performance or reliability, or both. If an MDisk group contains MDisks
with different levels of performance, the performance of the VDisks
in this group is limited by the performance of the slowest MDisk.
If an MDisk group contains MDisks with different levels of reliability,
the reliability of the VDisks in this group is that of the least reliable
MDisk in the group.
Note: When you create an
MDisk group with a new solid-state drive (SSD),
the new SSD is
automatically formatted and set to a block size of 512 bytes.
Even with the best planning, circumstances can change
and you must reconfigure your MDisk groups after they have been created.
The data migration facilities that are provided by the SAN Volume Controller enable
you to move data without disrupting I/O.
Choosing an MDisk
group extent size
Consider the following factors as
you decide the extent size of each new MDisk group:- You must specify the extent size when you create a new MDisk group.
- You cannot change the extent size later; it must remain constant
throughout the lifetime of the MDisk group.
- MDisk groups can have different extent sizes; however, this places
restrictions on the use of data migration.
- The choice of extent size affects
the maximum size of a VDisk in the MDisk group.
Table 1 compares
the maximum VDisk capacity for each extent size. The maximum is different
for
space-efficient VDisks.
Because the
SAN Volume Controller allocates
a whole number of extents to each VDisk that is created, using a larger
extent size might increase the amount of storage that is wasted at
the end of each VDisk. Larger extent sizes also reduces the ability
of the
SAN Volume Controller to
distribute sequential I/O workloads across many MDisks and therefore
can reduce the performance benefits of virtualization.
Table 1. Maximum VDisk capacity by extent
size| Extent size (MB) |
Maximum VDisk capacity in GB (not space-efficient VDisks) |
Maximum VDisk capacity in GB (space-efficient VDisks) |
|---|
| 16 |
2048 (2 TB) |
2000 |
| 32 |
4096 (4 TB) |
4000 |
| 64 |
8192 (8 TB) |
8000 |
| 128 |
16,384 (16 TB) |
16,000 |
| 256 |
32,768 (32 TB) |
32,000 |
| 512 |
65,536 (64 TB) |
65,000 |
| 1024 |
131,072 (128 TB) |
130,000 |
| 2048 |
262,144 (256 TB) |
260,000 |
Important: You can specify different extent
sizes for different MDisk groups; however, you cannot migrate VDisks
between MDisk groups with different extent sizes. If possible, create
all your MDisk groups with the same extent size.
Perform
the following steps to create an MDisk group:
Issue the svctask mkmdiskgrp CLI command
to create an MDisk group. The following is an example
of the CLI command you can issue to create an MDisk group:
svctask mkmdiskgrp -name maindiskgroup -ext 32
-mdisk mdsk0:mdsk1:mdsk2:mdsk3
where maindiskgroup is the name of
the MDisk group that you want to create, 32 MB
is the size of the extent you want to use, and mdsk0, mdsk1,
mdsk2, mdsk3 are the names of the four MDisks that you want
to add to the group.
You created and added MDisks to an MDisk group.
The following example provides a scenario
where you want to create an MDisk group, but you do not have any MDisks
available to add to the group. You plan to add the MDisks at a later
time. You use the svctask mkmdiskgrp CLI command
to create the MDisk group bkpmdiskgroup and later
used the svctask addmdisk CLI command to add mdsk4,
mdsk5, mdsk6, mdsk7 to the MDisk group.
- Issue svctask mkmdiskgrp -name bkpmdiskgroup -ext 32
where bkpmdiskgroup is the name of
the MDisk group that you want to create and 32 MB
is the size of the extent that you want to use.
- You find four MDisks that you want to add to the MDisk group.
- Issue svctask addmdisk -mdisk mdsk4:mdsk5:mdsk6:mdsk7
bkpdiskgroup
where mdsk4, mdsk5,
mdsk6, mdsk7 are the names of the MDisks that you want to
add to the MDisk group and bkpdiskgroup is the
name of the MDisk group for which you want to add MDisks.