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200 CHAPTER 6 / EXTERNAL MEMORY

RAID 0 FOR HIGH I/O REQUEST RATE In a transaction-oriented environment,
the user is typically more concerned with response time than with transfer rate. For an
individual I/O request for a small amount of data,the I/O time is dominated by the mo-
tion of the disk heads (seek time) and the movement of the disk (rotational latency).
In a transaction environment, there may be hundreds of I/O requests per sec-
ond. A disk array can provide high I/O execution rates by balancing the I/O load
across multiple disks. Effective load balancing is achieved only if there are typically
multiple I/O requests outstanding.This, in turn, implies that there are multiple inde-
pendent applications or a single transaction-oriented application that is capable of
multiple asynchronous I/O requests.The performance will also be influenced by the
strip size. If the strip size is relatively large, so that a single I/O request only involves
a single disk access, then multiple waiting I/O requests can be handled in parallel,
reducing the queuing time for each request.

RAID Level 1
RAID 1 differs from RAID levels 2 through 6 in the way in which redundancy is
achieved. In these other RAID schemes, some form of parity calculation is used to
introduce redundancy, whereas in RAID 1, redundancy is achieved by the simple
expedient of duplicating all the data.As Figure 6.8b shows, data striping is used, as in
RAID 0. But in this case, each logical strip is mapped to two separate physical disks
so that every disk in the array has a mirror disk that contains the same data. RAID
1 can also be implemented without data striping, though this is less common.
There are a number of positive aspects to the RAID 1 organization:
1. A read request can be serviced by either of the two disks that contains the
requested data, whichever one involves the minimum seek time plus rotational
latency.
2. A write request requires that both corresponding strips be updated, but this can
be done in parallel. Thus, the write performance is dictated by the slower of the
two writes (i.e.,the one that involves the larger seek time plus rotational latency).
However, there is no “write penalty” with RAID 1. RAID levels 2 through 6 in-
volve the use of parity bits. Therefore, when a single strip is updated, the array
management software must first compute and update the parity bits as well as
updating the actual strip in question.
3. Recovery from a failure is simple. When a drive fails, the data may still be ac-
cessed from the second drive.

The principal disadvantage of RAID 1 is the cost; it requires twice the disk
space of the logical disk that it supports. Because of that, a RAID 1 configuration is
likely to be limited to drives that store system software and data and other highly
critical files. In these cases, RAID 1 provides real-time copy of all data so that in the
event of a disk failure, all of the critical data are still immediately available.
In a transaction-oriented environment, RAID 1 can achieve high I/O request
rates if the bulk of the requests are reads. In this situation, the performance of
RAID 1 can approach double of that of RAID 0. However, if a substantial fraction
of the I/O requests are write requests, then there may be no significant performance
gain over RAID 0. RAID 1 may also provide improved performance over RAID 0