Information Technology Reference
In-Depth Information
Exercises
5. Go to an on-line site that sells hard disk drives, and find the largest capac-
ity disk you can buy for less than $200. Now, track down the spec sheet
for the disk and, given the disk's specied bit error rate (or unrecoverable
read rate), estimate the probability of encountering an error if you read
every sector on the disk once.
6. Suppose we dene a RAID's access cost as the number disk accesses di-
vided by the number of data blocks read or written. For each of following
configurations and workloads, what is the access cost?
(a) Workload: a series of random 1-block writes
Configuration: mirroring
(b) Workload: a series of random 1-block writes
Configuration: distributed parity
(c) Workload: a series of random 1-block reads
Configuration: mirroring
(d) Workload: a series of random 1-block reads
Configuration: distributed parity
(e) Workload: a series of random 1-block reads
Configuration:
distributed parity with groupsize G and one failed
disk
(f) Workload: a long sequential write
Configuration: mirroring
(g) Workload: a long sequential write
Configuration: distributed parity with a group size of G
7. Suppose that an engineer who has not taken this class tries to create a
disk array with dual-redundancy but instead of using an appropriate error
correcting code such as Reed Soloman, the engineer simply stores a copy
of each parity block on two disks. e.g.,
data0
data1
data2
data3
parity
parity
Give an example of how a two-disk failure can cause a stripe to lose data
in such a system. Explain why data cannot be reconstructed in that case.
8. Some RAID systems improve reliability with intra-disk redundancy to
protect against nonrecoverable read failures. For example, each individual
disk on such a system might reserve one 4KB parity block in every 32 KB
extent and then store 28KB (7 4KB blocks) of data and 4 KB (1 4KB
block) of parity in each extent.
