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Note: All the posts are based on practical approach avoiding lengthy theory. All have been tested on some development servers. Please don’t test any post on production servers until you are sure.

Tuesday, October 04, 2011

How to check if the IO of the Database is Slow

We will outline some of the thresholds whereby RDBMS Support may consider IO to be slow and thus a potential reason for a performance problem. If the underlying cause for slow performance is found to be a result of slow IO at the OS level, then the appropriate vendor responsible for the IO subsystem (hardware and software) should be engaged to diagnose and correct the situation

The efficiency of IO may be measured in 2 ways :
  • Response Time
    Measured in milliseconds an operation takes to complete. This statistic is gathered by Oracle.
  • Throughput
    Measured as the number of operations per unit of time. This is calculated using OS tools
    for example iostat on Unix.
We will concentrates on how to determine whether IO is slow from the perspective of Oracle only.

Response time

Hardware does not necessarily respond in a uniform fashion for each IO request; there are always likely to be peaks and troughs. It is therefore common to measure response time using an average.

Note: In order to mitigate the effects of high/low value anomalies, the size of the sampled data set needs to be 'significant'. To this end, the number of samples should be at least 1000 operations per hour in order to be considered reliable and useable for decision making.

Types of IO

The average response time is directly related to the type of IO:
  • Read or Write

  • Single Block or MultiBlock

    Single block IO, as suggested by its name, reads only one at a time.
    For example, when a session waits for a single-block IO it may typically wait on the event "db file sequential read" to indicate that it is waiting for that block to be delivered.

    Multi-Block operations read more than one block at a time ranging from 2 to 128 Oracle blocks depending on the size of the block and operating system settings. Usually a multi-block request had a limit of 1Mb in size.
    For example when a session waits for a multi-block IO it may typically wait on the event  "db file scattered read" to indicate that it is waiting for those blocks to be delivered.
  • Synchronous or Asynchronous.

    Synchronous (Blocking) operations wait for the hardware to complete the physical I/O, so that they can be informed of and manage appropriately the success or failure of the operation (and to receive the results in the case of a successful read). The execution of the process is blocked while it waits for the results of the system call.

    With Asynchronous (Non-Blocking) operations the system call will return immediately once the I/O request has been passed down to the hardware or queued in the operating system (typically before the physical I/O operation has even begun). The execution of the process is not blocked, because it does not need to wait for the results of the system call. Instead, it can continue executing and then receive the results of the I/O operation later, once they are available.

Expected thresholds for response time

A typical multi-block synchronous read of 64 x 8k blocks (512kB total) should have an average of at most 20 milliseconds before worrying about 'slow IO'.  Smaller requests should be faster (10-20ms) whereas for larger requests, the elapsed time should be no more than 25ms.

Asynchronous operations should be equally as fast as or faster than synchronous.
Single Block operations should be as fast as or faster than multi-block

'log file parallel write', 'control file write' and 'direct path writes' should be no more than 15ms.

Datafile writes are not as easy to measure as reads. DBWR writes blocks asynchronously in batch ('db file parallel write' ) and there is currently no criteria to sanction the response time.

Other wait events and statistics are used to reveal if DBWR (multiple or single, with or without IO slaves) is fast enough to clean the dirty blocks.

As a rule, times higher than those stated above should usually be investigated as should any change for the worse noticed when comparing with previous timings taken.

Note: Just because a system is below these maximum thresholds does not mean that there are no more available tuning opportunities.

Response Times will vary from system to system. As an example, the following could be considered an acceptable average:
10 ms for MultiBlock Synchronous Reads
  5 ms for SingleBlock Synchronous Reads
  3 ms for 'log file parallel write'
This is based on the premise that multiblock IO may require more IO subsystem work than a single block IO and that, if recommendations are followed, redo logs are likely to be on the fastest disks with no other concurrent activity.

Identifying  IO Response Time

Oracle records the response time of IO operations as the "Elapsed Time" indicated in  specific wait events and statistics."Response time" and "elapsed time" are synonymous and interchangeable terms in this context.

Below is a list of some of the more popular wait events and their typical acceptable wait times (not an exhaustive list)
Wait Event R/W Synchronous
Elapsed Time
(with 1000+ waits per hour)
control file parallel write
Asynchronous Multi < 15ms
control file sequential read
Synchronous Single < 20 ms
db file parallel read Read Asynchronous Multi < 20 ms
db file scattered read Read Synchronous Multi < 20 ms
db file sequential read Read Synchronous Single < 20 ms
direct path read Read Asynchronous Multi < 20 ms
direct path read temp Read Asynchronous Multi < 20 ms
direct path write Write Asynchronous Multi < 15 ms
direct path write temp Write Asynchronous Multi < 15 ms
log file parallel write Write Asynchronous Multi < 15 ms

Sources in Oracle identifying Response Time (How to enable it  see How To Collect 10046 Trace (SQL_TRACE) Diagnostics for Performance Issues)

0046 Trace File

When level 8 or 12 is specified in the 10046 trace, wait events are included.
The response time is specified in ela field.
From 9i onwards the value is specified in microseconds. In 8i and before the time is in is 1/100th second (10ms). 

WAIT #5: nam='cell single block physical read' ela= 672 cellhash#=2520626383 diskhash#=1377492511 bytes=16384 obj#=63 tim=1280416903276618

672 microseconds = 0.672 ms

WAIT #5: nam='db file sequential read' ela= 1018 file#=2 block#=558091 blocks=1 obj#=0 tim=10191852599110

1018 microseconds => 1.018 ms

System State Dump

For each process in a system state, the wait information is included among the other process information. This will either show an active wait:  "waiting for" or a case where waiting is completed and the process is on CPU : "waited for" / "last wait for".
  • "waiting for"
    This means that the process is currently in a wait state.
    Prior to 11g the field to look at is "seconds since wait started" which shows how long the process has been waiting on this wait event
    Starting 11gR1 the field to look at is "total" which is the total time elapsed on this wait.

    If  a process is indicated to be  "waiting for" an IO related operation and "seconds since wait started" > 0 most likely this means that the IO got "lost" and the session can be considered to be hanging. (since we have mentioned previously that an average acceptable wait would be 20ms , any IO wait of duration > 1 second is a cause for concern)
  • "last wait for" is relevant in versions prior to 11g and indicates a process that is no longer waiting (ie it is on CPU). The last wait is recorded and its wait time indicated in "wait_time" field. (In 11g, "last wait for" is replaced by "not in wait")

    last wait for 'db file sequential read' blocking sess=0x0 seq=100 wait_time=2264 seconds since wait started=0
    file#=45, block#=17a57, blocks=1

    2264 microseconds => 2.264 ms

 "waited for" means the session is no longer waiting. This is used in systemstate trace starting from 11gR1.The field to look at is "total" indicating the total time waited.

0: waited for 'db file sequential read' file#=9, block#=46526, blocks=1
wait_id=179 seq_num=180 snap_id=1
wait times: snap=0.007039 sec, exc=0.007039 sec, total=0.007039 sec
wait times: max=infinite
wait counts: calls=0 os=0

0.007039 sec => 7.039 ms

Statspack and AWR reports

Foreground and Background Wait Events
These reports show sections detailing waits by both foreground and background operations separately. The following is an example of such a section:

                                        %Time Total Wait    wait    Waits   % DB
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
db file sequential read       2,354,428     0      8,256       4      2.6   21.2
db file scattered read           23,614     0         48       2      0.0     .1

In this report the average Response Time is shown by the Av Rd (ms) column (Average Read in Milliseconds)

Tablespace IO Stats

The Tablespace section of these reports also gives useful information from the tablespace perspective:

                 Av       Av     Av                       Av     Buffer  Av Buf 
         Reads Reads/s  Rd(ms) Blks/Rd       Writes Writes/s      Waits  Wt(ms) 
-------------- ------- ------- ------- ------------ -------- ---------- ------- 
     1,606,553     446     2.2     8.3       75,575       21     60,542     0.9 

Again the Read Response Time is indicated by the Av Rd (ms) column (Average Read in Milliseconds). The write IO time is indicated by the amount of time it is having to wait to service buffer writes which is indicated in the report by the Av Buf Wt(ms) column (Average time to write a buffer in Milliseconds)
Wait Event Histogram
The Wait event histogram section can provide useful information regarding the spread of the write times that makes up the average. It can show if the average is made up of many writes close to the average or if it is being skewed by a few very large or small values:

% of Waits                  
Event                      Waits  <1ms  <2ms  <4ms  <8ms <16ms <32ms  <=1s   >1s
-------------------------- ----- ----- ----- ----- ----- ----- ----- ----- -----
db file parallel read       4139    .2    .5   2.5  26.4  23.5  15.0  31.9    .1
db file parallel write      329K  88.5   4.0   2.1   1.9   2.3   1.1    .3    .0
db file scattered read     14.4K  54.3   8.5   6.1  16.6  11.5   2.6    .4      

Each column indicates the percentage of wait events waited up to the time specified between each bucket. For example the waits indicated under "<16ms" are those that are greater than "8ms" but less than "16ms".
As long as the greatest percentage of waits are in the buckets from <1ms up to 16ms then the IO performance is generally acceptable.

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