Database Reference
In-Depth Information
created for each data file, while the checking interval values throughout the expected
storage duration of the data file are calculated. This algorithm is essentially the imple-
mentation of the optimized version of the minimum replication approach described in
Chapter 5
. To facilitate the practical application of the approach, the equation in the
algorithm has been revised. In the original minimum replication calculation approach,
the longest storage duration of a data file for meeting the data reliability requirement
is predicted by following
equation (5.8)
, where
RA
(1
k
is the data reliability assurance
that is not yet determined. In the minimum replication algorithm, the calculation of
checking interval follows equation
(6.1)
:
(1)
1 1
=
−− −
e
−
λ
t
)(1
e
−
λ
t
)
1
2
(6.1)
RR
t
By revising the equation, the checking interval can be determined before
RA
(1
k
is determined. As
RR
(1)
is the data reliability requirement provided by the storage
user, the output of the algorithm, that is, checking the interval of the data file, can still
ensure that the data reliability requirement is not jeopardized.
Figure 6.3
shows the
pseudo code of the minimum replication algorithm.
In
Figure 6.3
,
ET
is the expected storage duration of the data file.
P
1
and
P
2
are
the disk failure rate patterns of the two disks for storing two replicas of the data file.
StartT
is the time when the algorithm starts.
CIS
is the result set containing all check-
ing interval values. The algorithm first calculates the average failure rate of the data
file stored on
disk 1
for the duration of
ET
(line 1). According to this value and inequa-
tion
(5.6)
, it determines the number of replicas that needs to be stored, that is, to store
the data file with one replica or two (line 2). If storing the data file with two replicas
is necessary, based on the piecewise functions for the variable failure rate patterns of
disk
1 and
disk
2, the algorithm calculates all checking interval values throughout the
Figure 6.3 Pseudo code of the minimum replication algorithm
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