Do You Know If Your Database Is Slow?

The time to respond:

There was a question at Pythian a while ago on how to monitor Oracle database instance performanceand alert if there is significant degradation. That got me thinking, while there are different approaches that different DBAs would take to interactively measure current instance performance, here we would need something simple. It would need to give a decisive answer and be able to say that “current performance is not acceptable” or “current performance is within normal (expected) limits”.

Going to the basics of how database performance can be described, we can simply say that database performance is either the response time of the operations the end-user do and/or the amount of work the database instance does in a certain time period – throughput.

We can easily find these metrics in from the v$sysmetric dynamic view:

SQL> select to_char(begin_time,'hh24:mi') time, round( value * 10, 2) "Response Time (ms)"
from v$sysmetric
where metric_name='SQL Service Response Time
'

TIME Response Time (ms)
--------------- ------------------
07:20 .32


So this is the last-minute response time for user calls (here in ms). We can check the throughput by checking the amount of logical blocks (it includes the physical blocks) being read, plus we can add direct reads (last minute and last several seconds output here for a database with 8 KB block):

SQL> select a.begin_time, a.end_time, round(((a.value + b.value)/131072),2) "GB per sec"

from v$sysmetric a, v$sysmetric b

where a.metric_name = 'Logical Reads Per Sec'

and b.metric_name = 'Physical Reads Direct Per Sec'

and a.begin_time = b.begin_time


BEGIN_TIME END_TIME GB per sec

-------------------- -------------------- ----------

16-jun-2013 08:51:36 16-jun-2013 08:52:37 .01

16-jun-2013 08:52:22 16-jun-2013 08:52:37 .01


We can check more historical values through v$sysmetric_summary, v$sysmetric_history and dba_hist_ssysmetric_summary.

So did these queries answer the basic question “Do we have bad performance?”? 100 MB/sec throughput and 0.32 ms for a user call? We have seen better performance, but is it bad enough that we should alert the on-call DBA to investigate in more detail and look for the reason why we are seeing this kind of values? We cannot say. We need something to compare these values to so that we can determine if they are too low or too high. It is somewhat like being in a train that passes next to another moving train, going in same direction but at a different speed. We don’t know the speed of our train, and we don’t know the speed of the other train, so we cannot answer the question “Are we going very fast?”. If we turn to the other side and see a tree passing on the other side of the train, we will be able to estimate the speed of the train (also taking into account our experience of what is very fast for a train…). So we need something that has an absolute value. In the case of the tree, we know that the tree has speed of 0

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