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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 416
SEC. 5.4
DISKS
385
recalibrations insert gaps into the bit stream and are unacceptable. Special drives,
called
AV disks
(
Audio Visual disks
), which never recalibrate are available for
such applications.
Anecdotally, a highly convincing demonstration of how advanced disk con-
trollers have become was given by the Dutch hacker Jeroen Domburg, who hacked
a modern disk controller to make it run custom code. It turns out the disk controller
is equipped with a fairly powerful multicore (!) ARM processor and has easily
enough resources to run Linux. If the bad guys hack your hard drive in this way,
they will be able to see and modify all data you transfer to and from the disk. Even
reinstalling the operating from scratch will not remove the infection, as the disk
controller itself is malicious and serves as a permanent backdoor. Alternatively,
you can collect a stack of broken hard drives from your local recycling center and
build your own cluster computer for free.
5.4.5 Stable Storage
As we have seen, disks sometimes make errors. Good sectors can suddenly be-
come bad sectors.
Whole drives can die unexpectedly.
RAIDs protect against a
few sectors going bad or even a drive falling out.
However, they do not protect
against write errors laying down bad data in the first place. They also do not pro-
tect against crashes during writes corrupting the original data without replacing
them by newer data.
For some applications, it is essential that data never be lost or corrupted, even
in the face of disk and CPU errors. Ideally, a disk should simply work all the time
with no errors.
Unfortunately, that is not achievable. What is achievable is a disk
subsystem that has the following property: when a write is issued to it, the disk ei-
ther correctly writes the data or it does nothing, leaving the existing data intact.
Such a system is called
stable storage
and is implemented in software (Lampson
and Sturgis, 1979).
The goal is to keep the disk consistent at all costs.
Below we
will describe a slight variant of the original idea.
Before describing the algorithm, it is important to have a clear model of the
possible errors. The model assumes that when a disk writes a block (one or more
sectors), either the write is correct or it is incorrect and this error can be detected
on a subsequent read by examining the values of the ECC fields. In principle,
guaranteed error detection is never possible because with a, say, 16-byte ECC field
guarding a 512-byte sector, there are 2
4096
data values and only 2
144
ECC values.
Thus if a block is garbled during writing but the ECC is not, there are billions upon
billions of incorrect combinations that yield the same ECC.
If any of them occur,
the error will not be detected.
On the whole, the probability of random data having
the proper 16-byte ECC is about 2
−
144
, which is small enough that we will call it
zero, even though it is really not.
The model also assumes that a correctly written sector can spontaneously go
bad and become unreadable. However, the assumption is that such events are so
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