Avago Technologies MegaRAID Fast Path Software - Manual (Page 338)

Page 338 LSI Corporation Confidential | July 2011
MegaRAID SAS Software User Guide Chapter 8: Configuration | Creating a New Storage Configuration
NOTE: BGI is supported only for RAID 5 and RAID 6 and not for any other RAID levels.
New RAID 5 virtual drives require at least five drives for a background initialization to
start. New RAID 6 virtual drives require at least seven drives for a background
initialization to start. If there are fewer drives, the background initialization does not
start.
 Strip size: Strip sizes of 8 KB, 16 KB, 32 KB, 64 KB, 128 KB, 256 KB, 512 KB, and 1024
KB are supported. The default is 64 KB. For more information, see the striping entry
in the Glossary.
 Read policy: Specify the read policy for this virtual drive:
— Always read ahead: Read ahead capability allows the controller to read
sequentially ahead of requested data and to store the additional data in cache
memory, anticipating that the data will be needed soon. This process speeds up
reads for sequential data, but there is little improvement occurs when accessing
random data.
— No read ahead: (the default) Disables the read ahead capability.
 Write policy: Specify the write policy for this virtual drive:
— Write Through: In this mode, the controller sends a data transfer completion
signal to the host when the drive subsystem has received all of the data in a
transaction. This option eliminates the risk of losing cached data in case of a
power failure.
— Always Write Back: In this mode, the controller sends a data transfer
completion signal to the host when the controller cache has received all of the
data in a transaction.
— Write Back with BBU: (the default) In this mode, the controller enables write
back caching when the battery backup unit (BBU) is installed and charged. This
option provides a good balance between data protection and performance.
NOTE: The write policy depends on the status of the BBU. If the BBU is not present, is
low, is failed, or is being charged, the current write policy switches to write through,
which provides better data protection.
 I/O policy: The I/O policy applies to reads on a specific virtual drive. It does not
affect the read ahead cache.
— Cached IO: In this mode, all reads are buffered in cache memory.
— Direct IO: (the default) In this mode, reads are not buffered in cache memory.
Data is transferred to the cache and the host concurrently. If the same data block
is read again, it comes from cache memory.
Cached IO provides faster processing, and Direct IO ensures that the cache and
the host contain the same data.
 Access policy: Select the type of data access that is allowed for this virtual drive.
— Read/Write: (the default) Allow read/write access. This setting is the default
value.
— Read Only: Allow read-only access.
— Blocked: Do not allow access.