Apple Xsan 1.x - Manual (Page 10)

10 Chapter 1 Setup and Tuning Guidelines
Choosing 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 against metadata loss.
Use RAID 5 for Data LUNs
Xserve RAID systems are optimized for excellent performance and data redundancy
using a RAID 5 scheme. (RAID 5 stripes data across the available drives and also
distributes parity data across the drives.) Xserve RAID systems ship already configured
as RAID 5 LUNs. RAID 0 (striping with no parity) might give slightly better write
performance but provides no data recovery protection, so RAID 5 is always a better
choice for LUNs used to store user data.
7-Drive LUN vs 6-Drive LUN With Hot Spare
For best performance, use full 7-drive RAID 5 LUNs. Even if a drive fails, the degraded
6-drive array will continue to provide excellent performance. Keep in mind, however,
that the array is unprotected against the loss (however unlikely) of a second drive until
someone replaces the original faulty drive.
If you can’t afford to have the LUN operating in a degraded state until someone
replaces the faulty drive, you can configure your Xserve RAID systems as 6-drive RAID 5
arrays and use the seventh drive as a hot spare. Data on the faulty drive is
reconstructed automatically without human intervention.
Create One LUN per Xserve RAID Controller
For high performance data sets, create only one LUN on each Xserve RAID controller
(one array on each side of the system). Xserve RAID systems ship with one RAID 5 array
on each controller.
Working With LUNs Larger Than 2 Terabytes
The capacity of an Xserve RAID array can exceed 2 terabytes (TB) if the system contains
large drive modules. However, Xsan can’t use a LUN that is larger than 2 TB. If you set
up your Xserve RAID systems as one array per controller, as suggested above, you can’t
take advantage of the array capacity beyond 2 TB. To use as much available space as
possible, you can move drive modules to other controllers or slice a large array into two
smaller (less than 2 TB) LUNs. Slicing an array might, however, slow SAN performance.
Note: For the best possible SAN performance, don’t slice an array to create multiple
LUNs on a single controller.