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March 27th, 2007 19:00
RAID Best Practices and Array Maintenance Tips by Steve K
RAID Best Practices and Array Maintenance Tips
Dell recommends using Open Manage™ Storage Management to administer and maintain internal and external disk arrays connected to your Poweredge Server.
Dell OpenManage™ Server Administrator Storage Management provides enhanced features for configuring a system's locally-attached RAID and non-RAID disk storage. Storage Management enables you to perform controller and enclosure functions for all supported RAID and non-RAID controllers and enclosures from a single graphical or command-line interface without requiring use of the controller BIOS utilities. The graphical interface is wizard-driven with features for novice and advanced users and detailed online help. The command-line interface is fully-featured and scriptable. Using Storage Management, you can protect your data by configuring data-redundancy, assigning hot spares, or rebuilding failed physical disks. You can also perform data-destructive tasks. All users of Storage Management should be familiar with their storage environment and storage management.
Note: Storage Management supports SCSI, SATA, ATA, and SAS but not fibre channel.
• Assure that properly qualified cables are being used
• Verify cables are not excessively bent
• Inspect cabling for cuts, exposed shielding
• If external storage is attached, verify that cable length is appropriate for installation
• Run monthly consistency checks on you arrays:
http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/vdprin.html#365937
• Upgrade firmware, drivers, and Open Manage Server Administrator in sync with releases
• Monitor System event logs and Open Manage Server Administrator alerts:
http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/stprhl.html#211738
• Protect your virtual disks with a Hot Spare:
http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/hotspr.html#205258 • Establish best practices for backup and recovery of data
• Capture system logs, including Open Manage Alert logs, surrounding array/disk failures to assist in recovery
• Document the exact circumstances and events that caused the failed state
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<ADMIN NOTE: Broken link has been removed from this post by Dell>
Patrol Read is a feature for identifying disk errors in order to avoid disk failures and data loss or corruption. The Patrol Read only runs on disks that are being used in a virtual disk or that are hot spares.
When possible, the Patrol Read corrects disk errors and restores the integrity of the data. The Patrol Read task runs in the background. When the Patrol Read mode is set to Auto, the Patrol Read is initiated when the controller has been idle for a period of time and when no other background tasks are active. In this circumstance, the Patrol Read feature also enhances system performance as disk errors can be identified and corrected while the disk is not the subject of I/O activity.
The controller adjusts the amount of system resources dedicated to the Patrol Read based on the amount of controller activity that is competing with the Patrol Read task. During heavy controller activity, fewer system resources are dedicated to the Patrol Read task.
The Patrol Read will not run on a physical disk in the following circumstances:
* The physical disk is not included in a virtual disk or assigned as a hot spare.
* The physical disk is included in a virtual disk that is currently undergoing one of the following:
* A rebuild
* A reconfiguration or reconstruction
* A background initialization
* A check consistency
In addition, the Patrol Read suspends during heavy I/O activity and resumes when the I/O is finished.
To locate this task in Storage Management:
1. Expand the Storage tree object to display the controller objects.
2. Select a controller object.
3. Select the Information/Configuration subtab.
4. Select Set Patrol Read Mode from the Available Tasks drop-down menu.
5. Click Execute.
Use the Check Consistency task to verify the accuracy of the redundant (parity) information. This task only applies to redundant virtual disks. When necessary, the Check Consistency task rebuilds the redundant data. If the virtual disk is in a Failed Redundancy state, running a check consistency may be able to return the virtual disk to a Ready state.
A minimum recommendation is to perform consistency checks on each RAID logical drive once per month. This ensures that any media defects that may be encountered are repaired, and the data restored using parity data from the other members in the array (RAID 1, 5, 10 and 50). A consistency check also ensures that the bad media block is recorded in the grown defects list, and the bad block remapped to another location so that it is not used in the future on all RAID configurations.
Protecting your virtual disk with a Hot Spare
A hot spare is an unused backup physical disk that can be used to rebuild data from a redundant virtual disk. Hot spares remain in standby mode. When a physical disk that is used in a redundant virtual disk fails, the assigned hot spare is activated to replace the failed physical disk without interrupting the system or requiring your intervention. If a virtual disk using the failed physical disk is not redundant, then the data is permanently lost without any method (unless you have a backup) to restore the data.
Global Hot Spare:
A global hot spare is an unused backup disk that is part of the disk group. Hot spares remain in standby mode. When a physical disk that is used in a virtual disk fails, the assigned hot spare is activated to replace the failed physical disk without interrupting the system or requiring your intervention. When a hot spare is activated, it rebuilds the data for all redundant virtual disks that were using the failed physical disk.
http://support.dell.com/support/edocs/software/svradmin/5.1/en/omss_ug/html/adprin.html#323260
Dedicated Hot Spare:
A dedicated hot spare is an unused backup disk that is assigned to a single virtual disk. When a physical disk in the virtual disk fails, the hot spare is activated to replace the failed physical disk without interrupting the system or requiring your intervention.
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Dedicated Hot Spare Considerations
The following considerations apply to dedicated hot spares:
Considerations for RAID 10 and RAID 50.
If you have created a RAID 10 or RAID 50 virtual disk that does not fully consume its member physical disks, then you will not be able to assign a dedicated hot spare to the RAID 10 or RAID 50 virtual disk. Storage Management does not allow you to create RAID 10 and RAID 50 virtual disks from partial physical disks. You will therefore not encounter this situation if you are using Storage Management to create your virtual disks. If, however, the RAID 10 or 50 virtual disk was created using another application and if it does contain partial physical disks, then you will not be able to assign a dedicated hot spare to the virtual disk
.
Considerations for Multiple Dedicated Hot Spares.
Storage Management does not enable you to assign more than one dedicated hot spare to a virtual disk. On some controllers, you can use the BIOS to assign more than one dedicated hot spare. In this case, Storage Management recognizes the hot spares assigned in the BIOS but does not allow you to assign additional dedicated hot spares
RAID uses particular techniques for writing data to disks.
These techniques enable RAID to provide data redundancy or better performance.
These techniques include:
• Mirroring — Duplicating data from one physical disk to another physical disk. Mirroring provides data redundancy by maintaining two copies of the same data on different physical disks. If one of the disks in the mirror fails, the system can continue to operate using the unaffected disk. Both sides of the mirror contain the same data at all times. Either side of the mirror can act as the operational side. A mirrored RAID disk group is comparable in performance to a RAID 5 disk group in read operations but faster in write operations.
• Striping — Disk striping writes data across all physical disks in a virtual disk. Each stripe consists of consecutive virtual disk data addresses that are mapped in fixed-size units to each physical disk in the virtual disk using a sequential pattern. For example, if the virtual disk includes five physical disks, the stripe writes data to physical disks one through five without repeating any of the physical disks. The amount of space consumed by a stripe is the same on each physical disk. The portion of a stripe that resides on a physical disk is a stripe element. Striping by itself does not provide data redundancy. Striping in combination with parity does provide data redundancy.
• Stripe size— The total disk space consumed by a stripe not including a parity disk. For example, consider a stripe that contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe size is 64KB and the stripe element size is 16KB.
• Stripe element — A stripe element is the portion of a stripe that resides on a single physical disk.
• Stripe element size — The amount of disk space consumed by a stripe element. For example, consider a stripe that contains 64KB of disk space and has 16KB of data residing on each disk in the stripe. In this case, the stripe element size is 16KB and the stripe size is 64KB.
• Parity — Parity refers to redundant data that is maintained using an algorithm in combination with striping. When one of the striped disks fails, the data can be reconstructed from the parity information using the algorithm.
• Span — A span is a RAID technique used to combine storage space from groups of physical disks into a RAID 10 or 50 virtual disk.
Steve K is a Lead for the Enterprise Early Life Support Server Team and has been with Dell since January 2002. In his spare time, Steve enjoys cooking, computer games, and playing soccer.
Message Edited by DELL-Siobhan on 03-29-2007 02:53 PM
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April 30th, 2007 14:00