Dell PowerVault MD3260 Administrator's Guide - Page 22

Cycle Time, Virtual Disk Operations Limit, Disk Group Operations, RAID Level Migration

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Cycle Time The media verification operation runs only on selected disk groups, independent of other disk groups. Cycle time is the time taken to complete verification of the metadata region of the disk group and all virtual disks in the disk group for which media verification is configured. The next cycle for a disk group starts automatically when the current cycle completes. You can set the cycle time for a media verification operation between 1 and 30 days. The storage controller throttles the media verification I/O accesses to disks based on the cycle time. The storage array tracks the cycle for each disk group independent of other disk groups on the controller and creates a checkpoint. If the media verification operation on a disk group is preempted or blocked by another operation on the disk group, the storage array resumes after the current cycle. If the media verification process on a disk group is stopped due to a RAID controller module restart, the storage array resumes the process from the last checkpoint. Virtual Disk Operations Limit The maximum number of active, concurrent virtual disk processes per RAID controller module installed in the storage array is four. This limit is applied to the following virtual disk processes: • Background initialization • Foreground initialization • Consistency check • Rebuild • Copy back If a redundant RAID controller module fails with existing virtual disk processes, the processes on the failed controller are transferred to the peer controller. A transferred process is placed in a suspended state if there are four active processes on the peer controller. The suspended processes are resumed on the peer controller when the number of active processes falls below four. Disk Group Operations RAID Level Migration You can migrate from one RAID level to another depending on your requirements. For example, fault-tolerant characteristics can be added to a stripe set (RAID 0) by converting it to a RAID 5 set. The MD Storage Manager provides information about RAID attributes to assist you in selecting the appropriate RAID level. You can perform a RAID level migration while the system is still running and without rebooting, which maintains data availability. Segment Size Migration Segment size refers to the amount of data (in kilobytes) that the storage array writes on a physical disk in a virtual disk before writing data on the next physical disk. Valid values for the segment size are 8 KB, 16 KB, 32 KB, 64 KB, 128 KB, and 256 KB. Dynamic segment size migration enables the segment size of a given virtual disk to be changed. A default segment size is set when the virtual disk is created, based on such factors as the RAID level and expected usage. You can change the default value if segment size usage does not match your needs. When considering a segment size change, two scenarios illustrate different approaches to the limitations: 22

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Cycle Time
The media verification operation runs only on selected disk groups, independent of other disk groups. Cycle time is the
time taken to complete verification of the metadata region of the disk group and all virtual disks in the disk group for
which media verification is configured. The next cycle for a disk group starts automatically when the current cycle
completes. You can set the cycle time for a media verification operation between 1 and 30 days. The storage controller
throttles the media verification I/O accesses to disks based on the cycle time.
The storage array tracks the cycle for each disk group independent of other disk groups on the controller and creates a
checkpoint. If the media verification operation on a disk group is preempted or blocked by another operation on the disk
group, the storage array resumes after the current cycle. If the media verification process on a disk group is stopped
due to a RAID controller module restart, the storage array resumes the process from the last checkpoint.
Virtual Disk Operations Limit
The maximum number of active, concurrent virtual disk processes per RAID controller module installed in the storage
array is four. This limit is applied to the following virtual disk processes:
•
Background initialization
•
Foreground initialization
•
Consistency check
•
Rebuild
•
Copy back
If a redundant RAID controller module fails with existing virtual disk processes, the processes on the failed controller
are transferred to the peer controller. A transferred process is placed in a suspended state if there are four active
processes on the peer controller. The suspended processes are resumed on the peer controller when the number of
active processes falls below four.
Disk Group Operations
RAID Level Migration
You can migrate from one RAID level to another depending on your requirements. For example, fault-tolerant
characteristics can be added to a stripe set (RAID 0) by converting it to a RAID 5 set. The MD Storage Manager provides
information about RAID attributes to assist you in selecting the appropriate RAID level. You can perform a RAID level
migration while the system is still running and without rebooting, which maintains data availability.
Segment Size Migration
Segment size refers to the amount of data (in kilobytes) that the storage array writes on a physical disk in a virtual disk
before writing data on the next physical disk. Valid values for the segment size are 8 KB, 16 KB, 32 KB, 64 KB, 128 KB, and
256 KB.
Dynamic segment size migration enables the segment size of a given virtual disk to be changed. A default segment size
is set when the virtual disk is created, based on such factors as the RAID level and expected usage. You can change the
default value if segment size usage does not match your needs.
When considering a segment size change, two scenarios illustrate different approaches to the limitations:
22