Page 261 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes
Table 7.6: Swap Map Entry (struct swapmap)

Element Meaning
sm_unct Number of threads using the page. When decremented to
zero, the swap page is free and the free pages linked list can
be updated.
sm_next Index of the next free page in the swapmap []. This is valid
only if sm_ucnt is zero; that means that this swapmap entry
is included in the linked list beginning with swaptab’s st_free
.

A linked list of free swap pages begin at the swaptab entry’s st_free and use
each free swapmap entry’s sm_next. When a page of swap is needed, the
kernel walks the structures (using the getswap() routine in vm_swalloc.c),
which calls other routines that actually locate the chunk, and so forth.

7.7 Overview of RAID Structure

Disk drives have continued to get smaller and cheaper, so it is now economically feasible to
attach a large number of disks to a computer system. Having a large number of disks in a
system presents opportunities for improving the rate at which data can be read or written, if
the disks are operated in parallel. Furthermore, this setup offers the potential for improving
the reliability of data storage, because redundant information can be stored on multiple disks.
Thus, failure of one disk does not lead to loss of data. A variety of disk-organization techniques,
collectively called redundant arrays of inexpensive disks (RAID), are commonly used to address
the performance and reliability issues. In the past, RAIDS composed of small cheap disks were
viewed as a cost effective alternative to large, expensive disks; today, RAIDS are used for their
higher reliability and higher data-transfer rate, rather than for economic reasons. Hence, the I
in RAID stands for “independent”, instead of “inexpensive.”
7.7.1 Improvement of Reliability via Redundancy
Let us first consider reliability. The chance that some disk out of a set of N disks will fail is
much higher than the chance that a specific single disk will fail. Suppose that the mean time to
failure of a single disk is 100,000 hours. Then, the mean time to failure of some disk in an array
of 100 disks will be 100,000/100 = 1,000 hours, or 41.66 days, which is not long at all! If we
store only one copy of the data, then each disk failure will result in loss of a significant amount
of data-such a high rate of data loss is unacceptable. The solution to the problem of reliability
is to introduce redundancy; we store extra information that is not needed normally, but that
can be used in the event of failure of a disk to rebuild the lost information. Thus, even if a disk
fails, data are not lost. The simplest (but most expensive) approach to introducing redundancy
is to duplicate every disk. This technique is called mirroring (or shadowing). A logical disk then
consists of two physical disks, and every write is carried out on both disks. If one of the disks
fails, the data can be read from the other. Data will be lost only if the second disk fails before
the first failed disk is replaced.

The mean time to failure-where failure is the loss of data-of a mirrored disk depends on two
factors: the mean time to failure of the individual disks, as well as on the mean time to repair,
which is the time it takes (on average) to replace a failed disk and to restore the data on it.
Suppose that the failures of the two disks are independent; that is, the failure of one disk is not
connected to the failure of the other. Then, if the mean time to failure of a single disk is 100,000
hours and the mean time to repair is 10 hours, then the mean time to data loss of a mirrored
disk system is 100, 0002/(2 * 10) = 500 * l06 hours, or 57,000 years!

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