Information Technology Reference
In-Depth Information
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.
In this arrangement, each data block is protected by two parity blocks:
one interdisk parity block on a different disk and on intradisk parity block
on the same disk.
This approach may reduce a disk's eective nonrecoverable read error
rate because if one block in an extent is lost, it can be recovered from the
remaining sectors and parity on the disk. Of course, if multiple blocks in
the same extent are lost, the system must rely on redundancy from other
disks.
(a) Assuming that a disk's nonrecoverable read errors are independent
and occur at a rate of one lost 512 byte sector per 10
15
bits read,
what is the effective nonrecoverable read error rate if the operating
system stores one parity block per seven data blocks on the disk?
Hint: You may find the
bc
or
dc
arbitrary-precision calculators use-
ful. These programs are standard in many Unix, Linux, and OSX
distributions. See the man pages for instructions.
(b) Why is the above likely to significantly overstate the impact of intra-
disk redundancy?
9. Many RAID implementations allow on-line repair in which the system
continues to operate after a disk failure, while a new empty disk is inserted
to replaced the failed disk, and while regenerating and copying data to the
new disk.
Sketch a design for a 2-disk, mirrored RAID that allows the system to
remain on-line during reconstruction, while still ensuring that when the
data copying is done, the new disk is properly reconstructed (i.e., it is an
exact copy of other disk.)
In particular, specify (1) what is done by a recovery thread, (2) what is
done on a read during recovery, and (3) what is done on a write during
