Service manual

Sizing Disk Subsystems
92 Sun ONE Directory Server Installation and Tuning Guide • June 2003
RAID 0, Striped Volume
Striping spreads dataacrossmultiple physicaldisks. The logical disk, or volume,is
divided into chunks or stripes and then distributed in a round-robin fashion on
physical disks. A stripe is always one or more disk blocks in size,with all stripes
havingthesamesize.
The name RAID 0 is a contradiction in that it provides no redundancy. Any disk
failure in a RAID 0 stripe causes the entire logical volume to be lost. RAID 0 is,
however, the least expensive of all RAID levels as all disks are dedicated to data.
RAID 1, Mirrored Volume
Thepurposeofmirroringistoprovideredundancy.Ifoneofthedisksinthemirror
fails then the data remains available and processing may continue. The trade off is
that each physical disk is mirrored, meaning that half the physical disk space is
devoted to mirroring.
RAID 1+0
Also known as RAID 10, RAID 1+0 provides the highestlevels of performanceand
resiliency. Consequently, it is the most expensive level of RAID to implement. Data
continues to remain available after up to three disk failures as long as all of the
disks that fail form different mirrors. RAID 1+0 is implementedas a striped array
where segments are RAID 1.
RAID 0+1
RAID 0+1 is slightly less resilient than RAID 1+0. A stripe is created and then
mirrored. If one or more disks fails on the same side of the mirror, then the data
remains available. If a disk then fails on the other side of the mirror, however, the
logical volume is lost. This subtle difference with RAID 1+0 means disks on either
side can fail simultaneously yet data remains available. RAID 0+1 is implemented
as a mirrored array where segments are RAID 0.
RAID 5
RAID 5 is not as resilient as mirroring yet nevertheless provides redundancy in
that data remains available after a single disk failure. RAID 5 implements
redundancy using a parity stripe created by performing logical exclusive or on
bytes of corresponding stripes on other disks. When one disk fails, data for that
disk is recalculated using the data and parity in the corresponding stripes on the
remaining disks. Performance suffers however when such corrective calculations
must be performed.