Kajile One may also specify -q k size in Kbytes or -q m size in Mbytes or -q g size in Gbytes. Iozone —a If you wish to generate graphs then you may wish to turn on Excel mode. The place on the lower right that touches the floor of the graph is not actual data. Only available on some platforms. Each line has three fields. Additional notes on how to make the graphs Iozone sends Excel compatible output to standard out.
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One will always need to specify 0 so that any of the following tests will have a file to measure. Tells the VXFS filesystem that all operations to the file are to bypass the buffer cache and go directly to disk.
The stride read test will read records at this stride. See also -X and -Y for other options to control compute delay. When running throughput tests this option allows the user to specify the least number of processes or threads to start.
This option should be used in conjunction with the -u option. Tells Iozone the processor cache line size. This is used internally to help speed up the test. Some applications read into a single buffer over and over. Others have an array of buffers. This option allows both types of applications to be simulated. This option allows one to override the default and to use multiple internal buffers. This forces all writes to the file to go completely to disk before returning to the benchmark.
Give results in operations per second. Iozone will allocate another internal buffer that is aligned to the same processor cache boundary and is of a size that matches the processor cache. It will zero fill this alternate buffer before beginning each test. This will purge the processor cache and allow one to see the memory subsystem without the acceleration due to the processor cache.
Only available on some platforms. The first sub process or thread will begin on the specified processor. Future processes or threads will be placed on the next processor. Once the total number of cpus is exceeded then future processes or threads will be placed in a round robin fashion. One may also specify -q k size in Kbytes or -q m size in Mbytes or -q g size in Gbytes.
See —y for setting minimum record size. Iozone will create latency versus offset data files that can be imported with a graphics package and plotted. This is useful for finding if certain offsets have very high latencies. Such as the point where UFS will allocate its first indirect block. One can see from the data the impacts of the extent allocations for extent based filesystems with this option. One may also specify -r k size in Kbytes or -r m size in Mbytes or -r g size in Gbytes.
Iozone will generate an Excel compatible report to standard out. This file may be imported with Microsoft Excel space delimited and used to create a graph of the filesystem performance. Note: The 3D graphs are column oriented. You will need to select this when graphing as the default in Excel is row oriented data.
One may also specify -s k size in Kbytes or -s m size in Mbytes or -s g size in Gbytes. This tells Iozone the size of the processor cache. It is used internally for buffer alignment and for the purge functionality.
When running throughput tests this option allows the user to specify the greatest number of processes or threads to start. This option should be used in conjunction with the -l option. Iozone will unmount and remount this mount point before beginning each test. This guarantees that the buffer cache does not contain any of the file under test. Leave them present in the filesystem. Stonewalling is a technique used internally to Iozone. It is used during the throughput tests.
The code starts all threads or processes and then stops them on a barrier. Once they are all ready to start then they are all released at the same time. This ensures that the entire measurement was taken while all of the processes or threads were running in parallel. This flag allows one to turn off the stonewalling and see what happens. This option is useful if one has taken a system call trace of the application that is of interest.
One may also specify -y k size in Kbytes or -y m size in Mbytes or -y g size in Gbytes. See —q for setting maximum record size.
The file contains triplets of information: Byte offset, size of transfer, compute delay in milliseconds. Normally Iozone omits testing of small record sizes for very large files when used in full automatic mode. This option forces Iozone to include the small record sizes in the automatic tests also.
What can I see: The following are some graphs that were generated from the Iozone output files. Also note that the processor cache effects can be seen for file sizes of 16 Kbytes to 1Mbyte.
The graph above is displaying the impact of re-reading a file. Notice that the processor cache is now very important and causes the sharp peak. The next plateau to the right is buffer cache and finally above MB the file no longer fits in the buffer cache and real spindle speeds can be seen.
The graph above was created by running Iozone multiple times and then graphing the combination of the results. Here the graph is showing the throughput performance as a function of processes and number of disks participating in a filesystem. Not all platforms scale so well. The graph above shows single stream performance where file size and request size are changed. The place on the lower right that touches the floor of the graph is not actual data. Excel graphs empty cells as containing a zero.
This run was taken with the —a option. If one used the —A option then the area that was not tested would have been tested and had real values. Normally this is not a desirable area to test because it is very time consuming to write a MB file in 4k transfer sizes.
The —a option in Iozone tells Iozone to discontinue use of transfer sizes less than 64k once the file size is 32MB or bigger. This saves quite a bit of time. Notice the ridge that runs from the top left to the lower right down the center of the graph. This is where the request size fits in the processor cache.
For file sizes less than the size of the processor cache you can see the rise in performance as well. When both the file size and the transfer size is less than the processor cache it rises even higher. Although interesting to see, it is unlikely that you will be able to get applications to never write files that are bigger than the processor cache J However it might be possible to get applications to try to re-use buffers and keep the buffer size smaller than the processor cache size.
Here one can see that there are some file sizes and some record sizes that have very bad performance. Notice the performance dip at record sizes of Kbytes. The dropoff for files greater than 8MB is very interesting since this machine has 16 GB of memory and an 8GB buffer cache. This is a classic example of tuning for a specific application.
If the poor system administrator ever installs an application that likes to read or write files in a record size of Kbytes to 1Mbytes his users will probably take him out back for a conference. If the system would have been characterized before it was purchased it would never have made it into the building.
Another type of graph that can be produced is the Latency graph. When the -Q option is used Iozone will generate four. These are read offset latency, write offset latency, rewrite offset latency and reread offset latency. These files can be imported into Excel and then graphed. The latency versus offset information is useful for seeing if there are any particular offsets in a file that have high latencies.
These high latencies can be caused by a variety of causes. An example would be if the file size is just a bit bigger than the buffer cache size. The first time the file is written the latency will be low for each transfer. This is because the writes are going into the buffer cache and the application is allowed to continue immediately. The second time the file is written the latencies will be very high.
This is due to the fact that the buffer cache is now completely full of dirty data that must be written before the buffer can be reused. The reason that this occurs when the file is bigger than the buffer cache is because the write to the first block on the rewrite case will not find the block in the buffer cache and will be forced to clean a buffer before using it.
The cleaning will take time and will cause a longer latency for the write to complete. Another example is when the filesystem is mounted from a remote machine.
The latency graphs can help to identify high latencies for files that are being accessed over the network. In the re-read latency graph one can clearly see the client side cache that is in NFS Version 3. The reread latencies are clearly not the latencies that one would get if the reads actually went to the NFS server and back. Run rules: If you wish to get accurate results for the entire range of performance for a platform you need to make sure that the maximum file size that will be tested is bigger than the buffer cache.
In general you should be able to see three or four plateaus. File size fits in processor cache.