faq

linux下查看系统工具原码,如IOSTAT等

that the relevant kernel counters have been reinitialized...
You can install the 'sysstat' script by hand in the relevant startup
directory, or you can ask sysstat to do it for you: answer 'y' to the
question "Set crontab to start sar automatically" during config stage
('make config'), then compile sysstat as usual and run 'make install'
as the last stage.

~~~

2.8. My database (e.g. MySQL) doesn't appear to understand the time zone
displayed by 'sadf -d'...

The format includes the timezone detail in the output. This is to make
sure it is communicated clearly that UTC is how the data is always
converted and printed. Moreover we don't depend on the TZ environment
variable and we don't have some data converted to a different timezone
for any reason, known or unknown.
When you deal with raw accounting data you always want it in UTC.
Of course, you want it to all be the same when loading into a database.
If your database can't deal with timezones, you should write a short script
to strip the "UTC" characters from the data being loaded into the database.

~~~

2.9. I tried to use the -p option of the sadf command, together with the
options -s and -e. Unfortunately, I have nothing displayed at all.

This is because no data belong to the specified time interval!
In fact, -p option makes sadf display its timestamp as a UTC value
(Coordinated Universal Time), as indicated in the manual page. The UTC value
may be different from the value that sar (or sadf without option -p) displays.
The same remark applies to the use of -d option.

~~~

2.10. I cannot see all my disks when I use the sar -d command...

See question 3.1 below.

~~~

2.11. Do you know a tool which can graphically plot the data collected by sar?

Several such tools are lying around on the internet. I haven't tested all of
them and there must still be some way for improvement...
You can find among others: isag (a Perl script included in the sysstat
package), sarvant, sar2gp, loadgraph, SysStat Charts...
I've also heard of commercial tools which use sysstat: PerfMan comes to mind,
among others.
If you find others which you think are of real interest, please let me know
so that I can update this list.

~~~

2.12. When I launch sadc, I get the error message:
flock: Resource temporarily unavailable

You are launching sadc using -L option. With this option, sadc tries to
get an exclusive lock on the output file. The above error message indicates
that another sadc process was running and had already locked the same output
file. Stop all sadc instances and try again.

~~~

2.13. I have sysstat setup to run via cron:
0 * * * * /usr/local/lib/sa/sa1 600 6 &
so that I get an activity report every 10 minutes.
When I use sar to get my output, there is no reading for 00:00:00. This
means that at midnight everynight there is a spike, or dip, in the graphs.
How should I run sysstat / sar so that I get a reading for 00:00:00?

Sysstat does get its data at midnight, but two data samples are needed to
display the values.
When there is a "file rotation" (beginning of a new day), sadc writes its data
at the end of the previous daily data file (/var/log/sa/sa<DD>) *and* at the
beginning of the new one (/var/log/sa/sa<DD+1>). Please note that '-' must be
used to specify the output file for sadc to be able to detect such a file
rotation. So a crontab like the following one should enable you to get the
data for midnight at the end of each daily data file :
# Activity reports every 10 minutes from 01:00:00 to 22:50:00
0 1-22 * * * /usr/local/lib/sa/sa1 600 6 &
# Activity reports every 10 minutes from 23:00:00 to 00:00:00
# Reporting until 00:00:00 ensures that a file rotation will be detected
# by sadc
0 23 * * * /usr/local/lib/sa/sa1 600 7 &
# Activity reports every 10 minutes from 00:10:00 to 00:50:00
10 0 * * * /usr/local/lib/sa/sa1 600 5 &

~~~

2.14. The sar command complains with the following message:
Requested activities not available in file.

This error message means that you are trying to extract non-existent activities
from the data file. Usually sadc reads all the available activities from the
system and stores them in the data file. However, to prevent data files from
taking too much space on disk, some activities must be explicitly set on the
command line to be read by sadc. One example is -I option which must be used
together with sadc to be able to extract statistics for individual interrupts
from the data file. Another example is -d option for disks statistics.
Other activities cannot be saved into a file at all even if you specify the
relevant options on the command line. Process statistics (-x and -X options)
are such activities.
Please read sadc manual page to know exactly which activities may be extracted
from a data file.
NB: If the sar command complains with the error message:
"Requested activities not available" (without mentioning "in file"),
it means that you are trying to display activities that the kernel itself is
unable to provide.

~~~

2.15. Does sar need a lot of resources to run?

No, sar doesn't need a lot of CPU to run, nor does it make your system slow,
contrary to what some people think. In the first place, it only runs every ten
minutes by default. Secondly, when it does run, it is over and done very
quickly. Try:
time /usr/lib/sa/sa1
to verify that for yourself.
Nor do you have to be concerned about using up all your disk space.
sar will use about 220 kB for a whole day's worth of data, and it normally only
stores one week worth, and won't keep more than a month. It is entirely self
limiting. 

~~~

2.16. Are the measurements gathered by sadc cumulative or instantaneous values?

Each counter maintained by the kernel is cumulative since system boot. As a
consequence the measurements gathered by sadc are cumulative values.
Moreover all per-second statistics displayed by sar are average values on the
given time interval. So the value for counter foo at time T is calculated as:
foo/s = [foo(T) - foo(T-dt)] / dt
where dt is the interval given on the command line.

~~~

2.17. Some fields are always displayed as 0.00 when I use the sar -d
command.

See question 3.2 below.


3. QUESTIONS RELATING TO IOSTAT
###############################

3.1. I can't see all my disks when I use the iostat command...

Yes. This is a kernel limit. Old kernels (2.2.x for instance) used to
maintain stats for the first four devices.
The accounting code has changed in 2.4 kernels, and the result may (or
may not) be better for your system. Indeed, Linux 2.4 maintains the stats
in a two dimensional array, with a maximum of 16 devices (DK_MAX_DISK
in the kernel sources). Moreover, if the device major number exceeds
DK_MAX_MAJOR (whose value also defaults to 16 in the kernel sources),
then stats for this device will not be collected.
So, a solution may be simply to change the values of these limits in 
linux/include/linux/kernel_stat.h and recompile your kernel.
You should no longer have any problem with post 2.5 kernels, since
statistics are maintained by the kernel for all the devices.
In the particular case of iostat, also be sure to use the ALL keyword
on the command line to display statistical information relating to
every device, including those that are defined but have never been used
by the system.

~~~

3.2. iostat -x doesn't report disk I/O statistics...

For 'iostat -x' to be able to report extended disk I/O statistics,
it is better to use a recent version of the Linux kernel (2.6.x).
Indeed, iostat tries to read data from the /proc/diskstats file or
from the sysfs filesystem for that.
But iostat may also be able to display extended statistics with
older kernels (e.g. 2.4.x) providing that all the necessary
statistical information is available in the /proc/partitions file,
which requires that a patch be applied to the kernel (this is
often done on kernels included in various distros). In recent 2.4.x
kernels, the /proc/partitions file has all the necessary data
providing that the kernel has been compiled with CONFIG_BLK_STATS=y.

~~~

3.3. Why can't iostat display extended statistics for partitions with
     2.6.x kernels?

Because the kernel maintains these stats only for devices, and not for
partitions! Here is an excerpt from the document iostats.txt written by
Rick Lindsley ([e-mail]) and included in the kernel source
documentation:
"There were significant changes between 2.4 and 2.6 in the I/O subsystem.
As a result, some statistic information disappeared. The translation from
a disk address relative to a partition to the disk address relative to
the host disk happens much earlier.  All merges and timings now happen
at the disk level rather than at both the disk and partition level as
in 2.4.  Consequently, you'll see a different statistics output on 2.6 for
partitions from that for disks."

~~~

3.4. I don't understand the output of iostat. It doesn't match what I expect it
to be...

By default iostat displays I/O activity in blocks per second. With old
kernels (i.e. older than 2.4.x) a block is of indeterminate size and therefore
the displayed values are not useful.
With recent kernels (kernels 2.4 and later), iostat is now able to get disk
activities from the kernel expressed in a number of sectors. If you take a
look at the kernel code, the sector size is actually allowed to vary although
I have never seen anything other than 512 bytes.

~~~

3.5. Why values displayed by iostat are so different in the first report
     from those displayed in subsequent ones?

Probably because, as written in the manual page, the first report generated
by iostat concerns the time since system startup, whereas subsequent ones
cover only the time since the previous report (that is to say, the interval
of time entered on the command line).

~~~

3.6. iostat -x displays huge numbers for some fields...

Because of a Linux kernel bug, iostat -x may display huge I/O response times 
(svctm) and a bandwidth utilization (%util) of 100% for some devices. Indeed
these devices have a value for the field #9 (beginning after the device name)
in /proc/{partitions,diskstats} which is always different from 0, and even
negative sometimes. Yet this field should go to zero, since it gives the
number of I/Os currently in progress (it is incremented as requests are
submitted, and decremented as they finish).
To (temporarily) solve the problem, you should reboot your system to reset the
counters in /proc/{partitions,diskstats}.

--	
Sebastien Godard (sysstat <at> wanadoo.fr) is the author and the current
maintainer of this package.