Cache Configuration - HP P2000 G3 MSA Technical White Paper

Technical white paper | HP P2000 G3 MSA
Spares
You can designate a maximum of eight global spares for the system. If a disk in any redundant Vdisk (RAID 1, 3, 5, 6, 10, and
50) fails, a global spare is automatically used to reconstruct the Vdisk.
At least one Vdisk must exist before you can add a global spare. A spare must have sufficient capacity to replace the
smallest disk in an existing Vdisk. If a drive in the virtual disk fails, the controller automatically uses the Vdisk spare for
reconstruction of the critical virtual disk to which it belongs. A spare drive must be the same type (SAS or SATA) as other
drives in the virtual disk. You cannot add a spare that has insufficient capacity to replace the largest drive in the virtual disk.
If two drives fail in a RAID 6 virtual disk, two properly sized spare drives must be available before reconstruction can begin.
For RAID 50 virtual disks, if more than one sub-disk becomes critical, reconstruction and use of Vdisk spares occur in the
order sub-Vdisks are numbered.
You can designate a global spare to replace a failed drive in any virtual disk, or a Vdisk spare to replace a failed drive in only
a specific virtual disk. Alternatively, you can enable dynamic spares in HP SMU. Dynamic sparing enables the system to use
any drive that is not part of a virtual disk to replace a failed drive in any virtual disk.
Working with Failed Drives and Global Spares
When a failed drive rebuilds to a spare, the spare drive now becomes the new drive in the virtual disk. At this point, the
original drive slot position that failed is no longer part of the virtual disk. The original drive now becomes a "Leftover" drive.
In order to get the original drive slot position to become part of the virtual disk again, do the following:
1.
2.
3.
4.
5.
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7.
Tip
A best practice is to designate a spare disk drive for use if a drive fails. Although using a dedicated Vdisk spare is the best
way to provide spares for your virtual disks, it is also expensive to keep a spare assigned to each virtual disk. An alternative
method is to enable dynamic spares or to assign one or more unused drives as global spares.

Cache configuration

Controller cache options can be set for individual volumes to improve a volume's fault tolerance and I/O performance.
Note
To change the following cache settings, the user—who logs into the HP SMU—must have the "advanced" user credential.
The manage user has the "standard" user credential by default. This credential can be changed using the HP SMU and click
on "Configuration," then "Users," then "Modify Users."
Write-back cache settings
Write back is a cache-writing strategy in which the controller receives the data to be written to disk, stores it in the memory
buffer, and immediately sends the host operating system a signal that the write operation is complete, without waiting until
the data is actually written to the disk drive. Write-back cache mirrors all of the data from one controller module cache to
the other. Write-back cache improves the performance of write operations and the throughput of the controller.
When write-back cache is disabled, write-through becomes the cache-writing strategy. Using write-through cache, the
controller writes the data to the disk before signaling the host operating system that the process is complete. Write-through
cache has lower throughput and write operation performance than write back, but it is the safer strategy, with low risk of
data loss on power failure. However, write-through cache does not mirror the write data because the data is written to the
disk before posting command completion and mirroring is not required. You can set conditions that cause the controller to
switch from write-back caching to write-through caching as described in "Auto Write Through Trigger and Behavior Settings"
later in this paper.
Replace the failed drive with a new drive.
If the drive slot is still marked as "Leftover", use the "Clear Disk Metadata" option found in the "Tools" submenu.
When the new drive is online and marked as "Available", configure the drive as a global spare drive.
Fail the drive in the original global spare location by removing it from the enclosure. The RAID engine will rebuild to the
new global spare which will then become an active drive in the RAID set again.
Replace the drive you manually removed from the enclosure.
If the drive is marked as "Leftover", clear the metadata as in step 2 above.
Re-configure the drive as the new global spare.
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