Page 266 - DCAP103_Principle of operating system

Unit 7: Secondary Storage Structure

Due to RAID 1’s high space overhead, RAID level 5 is often preferred for storing large volumes Notes
of data. Level 6 is not supported currently by many RAID implementations, but it should offer
better reliability than level 5. RAID system designers have to make several other decisions as
well. For example, how many disks should be in an array? How many bits should be protected
by each parity bit? If more disks are in an array, data-transfer rates are higher, but the system
is more expensive. If more bits are protected by a parity bit, the space overhead due to parity
bits is lower, but the chance that a second disk will fail before the first failed disk is repaired
is greater, and that will result in data loss. One other aspect of most RAID implementations
is a hot spare disk or disks. A hot spare is not used for data, but is configured to be used as a
replacement should any other disk fail. For instance, a hot spare can be used to rebuild a mirror
pair should one of the disks in the pair fail. In this way, the RAID level can be reestablished
automatically, without waiting for the failed disk to be replaced. Allocating more than one hot
spare allows more than one failure to be repaired without human intervention.
7.8.2 Extensions
The concepts of RAID have been generalized to other storage devices, including arrays of tapes,
and even to the broadcast of data over wireless systems. When applied to arrays of tapes, the
RAID structures are able to recover data even if one of the tapes in an array of tapes is damaged.
When applied to broadcast of data, a block of data is split into short units and is broadcast along
with a parity unit; if one of the units is not received for any reason, it can be reconstructed from
the other units. Commonly, tape-drive robots containing multiple tape drives will stripe data
across all the drives to increase throughput and decrease backup time.

A block read accesses only one disk, allowing other requests to be processed
by the other disks. Thus, the data-transfer rate for each access is slower, but
multiple read accesses can proceed in parallel, leading to a higher overall
I/O rate. The transfer rates for large reads is high, since all the disks can be
read in parallel; large writes also have high transfer rates, since the data and
parity can be written in parallel.

Stable-storage Implementation

table storage is a classification of computer data storage technology that guarantees

atomicity for any given write operation and allows software to be written that is robust
Sagainst some hardware and power failures. To be considered atomic, upon reading back
a just written-to portion of the disk, the storage subsystem must return either the write data
or the data that was on that portion of the disk before the write operation. Most computer
disk drives are not considered stable storage because they do not guarantee atomic write:
an error could be returned upon subsequent read of the disk where it was just written to in
lieu of either the new or prior data.
Multiple techniques have been developed to achieve the atomic property from weakly-atomic
devices such as disks. Writing data to a disk in two places in a specific way is one technique
and can be done by application software. Most often though, stable storage functionality is
achieved by mirroring data on separate disks via RAID technology (level 1 or greater). The
RAID controller implements the disk writing algorithms that enable separate disks to act
as stable storage. The RAID technique is robust against some single disk failure in an array
of disks whereas the software technique of writing to separate areas of the same disk only
protects against some kinds of internal disk media failures such as bad sectors in single
disk arrangements.
Contd...

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