PlatinumNAS Owner’s Manual
RAID 10
A RAID 10 array is formed using a two-layer hierarchy of RAID types. At the lowest
level of the hierarchy are a set of RAID 1 arrays i.e., mirrored sets. These RAID 1 arrays in
turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective
result is a RAID 10 array. The figure below demonstrates a RAID 10 comprised of two RAID
1 arrays at the lower level of the hierarchy – arrays A and B. These two arrays in turn are
striped using 4 stripes (comprised of the strips 1A, 1B, 2A, 2B etc.) to form a RAID 0 at the
upper level of the hierarchy. The result is a RAID 10.
Advantage:
RAID 10 ensures that if
one of the disks in any parity group fails, its contents can be extracted using the information
on the remaining functioning disks in its parity group. Thus it offers better data redundancy
than the simple RAID types such as RAID 1, 3, and 5. Also, a RAID 10 array can improve the
throughput of read operations by allowing reads to be performed concurrently on multiple
disks in the set.
Read Ahead
Motivated by the principle of “spatial locality”, many RAID controllers read
blocks of data from secondary storage ahead of time, i.e., before an application actually
requests those blocks. The number of data blocks that are read ahead of time is typically
governed by some heuristic that observes the pattern of requests. The read-ahead technique
is particularly efficient when the spatial distribution of an application’s requests follows a
sequential pattern.
Read Around
When using this methodology, a read operation circumvents the cache and
directly reads data from secondary storage into system memory.
Read-Modify-Write
This is a term used to characterize an efficient methodology using
which parity is calculated and written into a RAID array. However, before we describe this
methodology, let us briefly touch upon the most obvious and brute-force way of determining
and writing parity (in response to a write operation) into a RAID array. Assume that data is
being written into a strip on the RAID array that supports redundancy by the use of parity. Let
us denote this as the target strip. Also assume that there are N strips per stripe including the
parity strip and the target strip. Then to recalculate parity for that stripe, the following steps
may be taken. First the contents of the N – 2 non-parity strips (belonging to the same stripe as
the target strip) have to be read. This is followed by N – 2 XOR operations on the contents of
the N – 2 strips that were just read plus the new contents of the target strip. This is followed by
1 operation to write the new data into the target strip and 1 operation to update the value of
the parity strip. In all the total number of read, XOR and write operations are N – 2, N – 2, and
2 respectively adding up to a grand total of 2N – 2. Let us now discuss the “read-modify-write”
method for calculating and writing parity. It is based on simple algebra, and is more efficient
than the method described earlier when the value of N is large. Suppose d1, d2,… dt,… dN-
1 are the data contents of the N – 1 non-parity strips with dt being the contents of the target
strip. Let, p = d1 ^ d2 ^ … dt ^ …^ dN-1 Now suppose that the new data to be written into
the target strip is d’t.We wish to determine the value of p’ = d1 ^ d2 ^ … d’t ^ …^ dN-1. Now,
p ^ p’ = (d1 ^ d2 ^ … dt ^ …^ dN-1) ^ (d1 ^ d2 ^ … d’t ^ …^ dN-1) p ^ p’ = dt ^ d’t, since ^
operation is commutative and associative p’ = dt ^ d’t ^ p In other words, the new parity can
be evaluated by calculating the XOR of the old data in the target strip, the new data for the
target strip and the old parity. Clearly this requires only 2 reads – one for the old data and the
old parity – followed by 2 XOR operations with 2 writes – one for writing the new data into
the target strip and the new parity, for a a grand total of 6 operations.
D-Glossary
