Network Servers: Everything You Wanted to Know but Were Afraid to Ask
If more than one drive is lost from a RAID 3 or a RAID 5 array, all the array's data will be
lost. Still, these arrays provide good protection at relatively low incremental cost.
This is another of the RAID standards that isn't used very much in
the real world. RAID 4 is similar to RAID 3, except data is striped between the
different data drives in much larger blocks than with RAID 3. RAID 4 still uses
a single ECC disk for all the data drives.
RAID 5, depicted in Figure 13-4, is the current standard for RAID
systems. (RAID 1 also remains a current standard, but it has different
applications.) Recall how RAID 3 worked, with data striped to a set of data
disks, and the ECC code written to a single ECC disk. RAID 5 improves on this
scheme by interleaving the data and ECC information across all the disks. The
big advantage of this approach over RAID 3 is that it doesn't rely on a single
ECC drive for all write operations, which becomes a bottleneck on RAID 3
systems. Because all the drives share the ECC work, performance with RAID 5
is slightly better than with RAID 3. There is a small drawback to this, though,
that most commentators miss. In RAID 3, if you lost a data drive, the system
slowed down (usually dramatically) as the data was reconstructed on the fly.
If you lost the ECC drive, however, the system would still run just as fast as if
no drive were lost. With RAID 5, if you lose a drive, you're always losing part
of your ECC drive (because its job is spread among all the disks), so you get a
slowdown no matter what.
RAID 6 works the same as RAID 5, but stores parity data on two
interleaving drives rather than the one of RAID 5. Also called dual data
guarding, RAID 6 keeps your data safe while you are recovering from a single
drive failure. (In RAID 5, if a drive failed while you were recovering from
another drive's failure, you would lose the array's data.)
A RAID-3 array stripes data across multiple disks, with an ECC disk to protect
Data striped on these drives
Last disk contains
ECC data for