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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 404
SEC. 5.4
DISKS
373
proper disks in the right sequence and then assemble the results in memory cor-
rectly. Performance is excellent and the implementation is straightforward.
RAID level 0 works worst with operating systems that habitually ask for data
one sector at a time. The results will be correct, but there is no parallelism and
hence no performance gain. Another disadvantage of this organization is that the
reliability is potentially worse than having a SLED.
If a RAID consists of four
disks, each with a mean time to failure of 20,000 hours, about once every 5000
hours a drive will fail and all the data will be completely lost.
A SLED with a
mean time to failure of 20,000 hours would be four times more reliable. Because
no redundancy is present in this design, it is not really a true RAID.
The next option, RAID level 1, shown in Fig. 5-20(b), is a true RAID.
It dupli-
cates all the disks, so there are four primary disks and four backup disks.
On a
write, every strip is written twice.
On a read, either copy can be used, distributing
the load over more drives. Consequently, write performance is no better than for a
single drive, but read performance can be up to twice as good. Fault tolerance is
excellent: if a drive crashes, the copy is simply used instead. Recovery consists of
simply installing a new drive and copying the entire backup drive to it.
Unlike levels 0 and 1, which work with strips of sectors, RAID level 2 works
on a word basis, possibly even a byte basis. Imagine splitting each byte of the sin-
gle virtual disk into a pair of 4-bit nibbles, then adding a Hamming code to each
one to form a 7-bit word, of which bits 1, 2, and 4 were parity bits. Further imagine
that the seven drives of Fig. 5-20(c) were synchronized in terms of arm position
and rotational position. Then it would be possible to write the 7-bit Hamming
coded word over the seven drives, one bit per drive.
The Thinking Machines CM-2 computer used this scheme, taking 32-bit data
words and adding 6 parity bits to form a 38-bit Hamming word, plus an extra bit
for word parity, and spread each word over 39 disk drives. The total throughput
was immense, because in one sector time it could write 32 sectors worth of data.
Also, losing one drive did not cause problems, because loss of a drive amounted to
losing 1 bit in each 39-bit word read, something the Hamming code could handle
on the fly.
On the down side, this scheme requires all the drives to be rotationally syn-
chronized, and it only makes sense with a substantial number of drives (even with
32 data drives and 6 parity drives, the overhead is 19%).
It also asks a lot of the
controller, since it must do a Hamming checksum every bit time.
RAID level 3 is a simplified version of RAID level 2.
It is illustrated in
Fig. 5-20(d).
Here a single parity bit is computed for each data word and written to
a parity drive.
As in RAID level 2, the drives must be exactly synchronized, since
individual data words are spread over multiple drives.
At first thought, it might appear that a single parity bit gives only error detec-
tion, not error correction. For the case of random undetected errors, this observa-
tion is true.
However, for the case of a drive crashing, it provides full 1-bit error
correction since the position of the bad bit is known. In the event that a drive
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