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Chapter 14. Monitoring and tuning z/VM and Linux 241

being measured in 100s rather than 1000s. The higher values seen tend to build up over

time, and are sustained over periods of intense system activity. However, there are times

when the MIGRATE value may spike for brief periods of time.

򐂰 Minidisk cache (MDC) statistics are given on the third line. The effectiveness of MDC can be

judged by the combination of the READS rate and the HIT RATIO. If both are high, then a

large number of physical I/Os are avoided due to the MDC feature. For a system that has

an appreciably high I/O rate, composed of reads plus writes, and a high proportion of

reads, and a good hit ratio for those reads (tending to 90% or greater), the real, physical

I/O avoidance can be high. This author has seen the avoidance as high as 50% in some

cases. Conversely, however, a high HIT RATIO with a low value for the READS rate should

not be taken as good (100% hit ratio, when doing only 1 I/O per second is effectively

meaningless).

򐂰 Line 4 describes more storage (memory) management. The PAGING rate is important.

Higher values will often impact performance. This can be at least partially offset by

increasing the number of page volumes, but a more thorough examination of this problem

is advisable whenever it arises.The STEAL percentage is often misleading. This is basically

the percentage of pages taken from guests that z/VM believes are non-dormant. Because

some guests have periodic timers going off, they appear to be active to z/VM even when

relatively idle. Pages taken from these guests are still considered to be stolen. So there

are scenarios where a system only has a user set comprising active guests, in which case

all pages taken would be considered stolen. Bearing this in mind, if a high STEAL value is

observed, the paging rate needs to be checked. If the paging rate is relatively low, then the

STEAL value is not important.

򐂰 On lines 5 through 8 you also see a series of counters that represent the users in various

queues. The z/VM scheduler classifies work into 3 different classes (1 through 3) and a

special additional class labelled zero. So the Column of Q

values and E

represent the

virtual machines in the dispatch list and the eligible list. The most important value here to

validate is that there are no virtual machines in the Eligible list: E1, E2, E3; this implies

z/VM has stopped dispatching some virtual machines to avoid overcommitting resources.

Such a system would require further investigation, possibly leading to some tuning work,

or even hardware addition in extreme cases. Ignore the values in parenthesis.

INDICATE QUEUES EXP

Another useful command to understand the state of the system is the INDICATE QUEUES

EXP command. Here is an example:

==> ind q exp

DATAMGT1 Q3 AP 00000537/00000537 .... -2.025 A02

BITNER Q1 R00 00000785/00000796 .I.. -1.782 A00

EDLLNX4 Q3 PS 00007635/00007635 .... -1.121 A00

TCPIP Q0 R01 00004016/00003336 .I.. -.9324 A01

APCTEST1 Q2 IO 00003556/00003512 .I.. -.7847 A01

EDLWRK20 Q3 AP 00001495/00001462 .... -.6996 A01

EDL Q3 IO 00000918/00000902 .... -.2409 A01

EDLWRK11 Q3 AP 00002323/00002299 .... -.0183 A00

EDLWRK18 Q3 IO 00001052/00000388 .... -.0047 A00

EDLWRK4 Q3 AP 00004792/00002295 .... .0055 A01

EDLWRK8 Q3 AP 00004804/00004797 .... .0089 A02

EDLWRK16 Q3 AP 00002378/00002378 .... .0170 A02

EDLWRK2 Q3 AP 00005544/00002956 .... .0360 A00

EDLWRK12 Q3 AP 00004963/00002348 .... .0677 A01

EDLWRK6 Q3 IO 00000750/00000302 .... .0969 A02

EDLWRK3 Q3 AP 00005098/00005096 .... .0999 A02

EDLWRK17 Q3 AP 00004786/00004766 .... .1061 A01

1 2 ... 256 257 258 259 260 261 262 263 264 265 266 ... 295 296

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