RAID5 on athlon64 machines

[Bakul Shah]

> 
> > Theoretically the sequential write rate should be same or
> > higher than the sequential read rate.  Given an N+1 disk
> 
> Seq write rate for the whole RAID5 array will always be lower
> than the write rate for it's single disk.

You compute max data rates by considering the most optimistic
scenario, which is large sequetial writes.  For *this*
situation write rate will be higher than a single disk's.

> The parity blocks are not read on data reads, since this would be
> unnecessary overhead and would diminish performance. The parity
> blocks are read, however, when a read of a data sector results
> in a cyclic redundancy check (CRC) error."

You can only do so if you know the array is consistent.  If
the system crashed there is no such guarantee.  So you either
have to rebuild the whole array to get to a consistent state
or do a parity check.  If you don't check parity and you have
an inconsistent array, you can have a silent error (the data
may be trashed but you don't know that).  But if you use RAM
without parity or ECC, you probably already don't care about
such errors.