Page 262 - DCAP103_Principle of operating system

Unit 7: Secondary Storage Structure

You should be aware that the assumption of independence of disk failures is not valid. Power Notes
failures and natural disasters, such as earthquakes, fires, and floods, may result in damage to
both disks at the same time. Also, manufacturing defects in a batch of disks can cause correlated
failures. As disks age, the probability of failure increases, increasing the chance that a second
disk will fail while the first is being repaired. In spite of all these considerations, however,
mirrored-disk systems offer much higher reliability than do single disk systems. Power failures
are a particular source of concern, since they occur far more frequently than do natural disasters.
However, even with mirroring of disks, if writes are in progress to the same block in both disks,
and power fails before both blocks are fully written, the two blocks can be in an inconsistent
state. The solution to this problem is to write one copy first, then the next, so that one of the
two copies is always consistent. Some extra actions are required when we restart after a power
failure, to recover from incomplete writes.

The simplest (but most expensive) approach to introducing redundancy is to
duplicate every disk. This technique is called mirroring (or shadowing). A logical
disk then consists of two physical disks, and every write is carried out on both
disks. If one of the disks fails, the data can be read from the other. Data will be
lost only if the second disk fails before the first failed disk is replaced.

7.7.2 Improvement in Performance via Parallelism
Now let us consider the benefit of parallel access to multiple disks. With disk mirroring, the rate
at which read requests can be handled is doubled, since read requests can be sent to either disk
(as long as both disks in a pair are functional, as is almost always the case). The transfer rate
of each read is the same as in a single-disk system, but the number of reads per unit time has
doubled. With multiple disks, we can improve the transfer rate as well (or instead) by striping
data across multiple disks. In its simplest form, data striping consists of splitting the bits of
each byte across multiple disks; such striping is called bit-level striping. For example, if we
have an array of eight disks, we write bit i of each byte to disk i. The array of eight disks can be
treated as a single disk with sectors that are eight times the normal size, and, more important,
that have eight times the access rate. In such an organization, every disk participates in every
access (read or write), so the number of accesses that can be processed per second is about the
same as on a single disk, but each access can read eight times as many data in the same time
as on a single disk. Bit-level striping can be generalized to a number of disks that either is a
multiple of 8 or divides 8. For example, if we use an array of four disks, bits I and 4+i of each
byte go to disk i. Further, striping does not need to be at the level of bits of a byte: For example,
in block-level striping, blocks of a file are striped across multiple disks; with n disks, block i
of a file goes to disk (i mod n) + 1. Other levels of striping, such as bytes of a sector or sectors
of a block, also are possible.
In summary, there are two main goals of parallelism in a disk system:

1. Increase the throughput of multiple small accesses (that is, page accesses) by load balancing.
2. Reduce the response time of large accesses.

7.8 RAID Levels
Mirroring provides high reliability, but it is expensive. Striping provides high data-transfer rates,
but it does not improve reliability. Numerous schemes to provide redundancy at lower cost by
using the idea of disk striping combined with “parity” bits (which we describe next) have been
proposed. These schemes have different cost-performance tradeoffs and are classified into levels
called RAID levels. We describe the various levels here; Figure, shows them pictorially (in the
figure, P indicates error-correcting bits and C indicates a second copy of the data). In all cases
depicted in the figure, four disks’ worth of data is stored, and the extra disks are used to store
redundant information for failure recovery.

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