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146 Storage Management with DB2 for OS/390
Figure 65. DB2 PM Accounting Highlights
12.1.3.2 Asynchronous Read I/O
B
in Figure 64 on page 145 shows 143669 asynchronous reads. This
corresponds to the sum of all prefetch operations (sequential, dynamic and list
prefetch). The suspend time for these reads is 29 minutes and 59.155952
seconds (
B in Figure 64). The total number of prefetch requests is not necessarily
equal to the total number of prefetch I/Os, which is reported at DB2 subsystem
level in the statistics record.
J in Figure 66 on page 147 shows that 5.94 million pages were read
asynchronously. The suspend time for reading these pages is 29 minutes and
59.155952 seconds (
B in Figure 64).This means that, on average, the program
had to wait 0.3 milliseconds for each page. The low buffer hit ratio (
E in Figure 66)
means that most pages had to be read from disk (or cache).
The total number of prefetch requests made by the query is
G + H + I.Inthis
example, it is 190714 requests. Of these, 143669 caused a suspension; see
B in
Figure 64. This means that 47045 (190714-143669) prefetch requests did not
cause the application program to wait; they have a response time of zero.
The average wait of all prefetch operations is the total wait time (
B in Figure 64)
divided by the total number of prefetch requests (
G + H + I). This is 9.4
milliseconds.
The number of pages read in each prefetch request can be calculated as the total
number of pages read asynchronously (
J in Figure 66 on page 147) divided by
the total number of prefetch requests (
G + H + I). In this example, this gives 31.2
pages (5943947/190714). This is very close to the maximum (32 pages) for these
buffer pools.
12.1.3.3 I/O Rate
The query did 19559 synchronous reads (F in Figure 66 on page 147) and
190714 prefetch requests (
G + H + I in Figure 66). This gives a total of 210273
I/O requests. For an interval of 37 minutes 40.4 seconds (
A in Figure 57 on page
142) this gives 93.0 I/O per second.
HIGHLIGHTS
--------------------------
THREAD TYPE : ALLIED
TERM.CONDITION: NORMAL
INVOKE REASON : DEALLOC
COMMITS : 2
ROLLBACK : 0
INCREM.BINDS : 0
UPDATE/COMMIT : 0.00
SYNCH I/O AVG.: 0.002903 D
PROGRAMS : 0
PARALLELISM : CP