Apple Xsan 2 - Manual (Page 54)

Connect RAID systems to an Ethernet network
For best performance, don’t connect RAID controller Ethernet management ports to
the SAN’s metadata network. Connect the ports to the separate Ethernet network that
you use for other types of network traffic, such as directory services, Internet access,
and remote Xsan management.
Choose RAID levels for LUNs
Use RAID 1 for metadata LUNs and RAID 5 for data LUNs.
Use RAID 1 for metadata LUNs
RAID 1 (mirroring) can give slightly better performance than the default RAID 5 scheme
for the small, two-drive metadata LUNs that Xsan uses to store volume information.
A single drive is almost always adequate for storing the primary volume metadata.
(10 GB of metadata space is enough for approximately 10 million files.) The second,
mirror drive protects you from metadata loss.
Use RAID 5 for data LUNs
Most RAID systems are optimized for excellent performance and data redundancy
using a RAID 5 scheme. (RAID 5 stripes data across available drives and distributes
parity data across the drives.) Some RAID systems ship preconfigured as RAID 5 LUNs.
RAID 0 (striping with no parity) might give slightly better write performance, but it
provides no data recovery protection, so RAID 5 is always a better choice for LUNs
used to store user data.
Adjust RAID system performance settings
RAID system performance settings, which affect parameters such as drive caching,
RAID controller caching, and read prefetching, can have a significant effect on Xsan
volume performance. Follow these guidelines.
Enable drive caching
In addition to the caching performed by the RAID controller, each drive in an array can
perform caching at the drive level to improve performance.
WARNING: If you enable drive caching for a RAID set, make sure the system is
connected to an uninterruptible power supply (UPS). Otherwise, you could lose
cached data if the power fails.
Enable RAID controller write caching
Without RAID controller write caching, a request to write data to the associated LUN
isn’t considered finished until the data is written to the physical disks that make up
the array. Only then can the next write request be processed. (This is sometimes called
write-through caching.)
54 Chapter 3 Plan a SAN