Table of Contents
Slackware Linux includes an implementation of the classic UNIX
cron daemon that allows users to schedule tasks for execution at
regular intervals. Each user can create, remove, or modify an
individual crontab file. This crontab file specifies commands
or scripts to be run at specified time intervals. Blank lines
in the crontab or lines that begin with a hash
(“#”) are ignored.
Each entry in the crontab file must contain 6 fields
separated by spaces. These fields are minute, hour, day, month,
day of week, and command. Each of the first five fields may
contain a time or the “*”
wildcard to match all times for that field. For example, to
have the date command run every day at 6:10
AM, the following entry could be used.
10 6 * * * date
A user crontab may be viewed with the crontab -l command. For a deeper introduction to the syntax of a crontab file, let us examine the default root crontab.
#crontab -l# If you don't want the output of a cron job mailed to you, you have to direct # any output to /dev/null. We'll do this here since these jobs should run # properly on a newly installed system, but if they don't the average newbie # might get quite perplexed about getting strange mail every 5 minutes. :^) # # Run the hourly, daily, weekly, and monthly cron jobs. # Jobs that need different timing may be entered into the crontab as before, # but most really don't need greater granularity than this. If the exact # times of the hourly, daily, weekly, and monthly cron jobs do not suit your # needs, feel free to adjust them. # # Run hourly cron jobs at 47 minutes after the hour: 47*
*
*
*
/usr/bin/run-parts /etc/cron.hourly 1> /dev/null
# # Run daily cron jobs at 4:40 every day: 40 4 * * * /usr/bin/run-parts /etc/cron.daily 1> /dev/null # # Run weekly cron jobs at 4:30 on the first day of the week: 30 4 * * 0 /usr/bin/run-parts /etc/cron.weekly 1> /dev/null # # Run monthly cron jobs at 4:20 on the first day of the month: 20 4 1 * * /usr/bin/run-parts /etc/cron.monthly 1> /dev/null
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The first field, |
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The second field, |
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The third field, |
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The fourth field, |
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The fifth field, |
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The sixth field, /usr/bin/run-parts /etc/cron.hourly 1> /dev/null, specifies the command that should be run at the time specification defined in the first five fields. |
The default root crontab is setup to run scripts in
/etc/cron.monthly/etc/cron.weekly/etc/cron.daily/etc/cron.hourly
Many modern disks offer various features for increasing disk performance and improving integrity. Many of these features can be tuned with the hdparm command. Be careful with changing disk settings with this utility, because some changes can damage data on your disk.
You can get an overview of the active settings for a disk by providing the device node of a disk as a parameter to hdparm:
# hdparm /dev/hda
/dev/hda:
multcount = 0 (off)
IO_support = 1 (32-bit)
unmaskirq = 1 (on)
using_dma = 1 (on)
keepsettings = 0 (off)
readonly = 0 (off)
readahead = 256 (on)
geometry = 65535/16/63, sectors = 78165360, start = 0
A common cause for bad disk performance is that DMA was not automatically used by the kernel for a disk. DMA will speed up I/O throughput and offload CPU usage, by making it possible for the disk to directly transfer data from the disk to the system memory. If DMA is turned off, the overview of settings would shows this line:
using_dma = 0 (off)
You can easily turn on DMA for this disk with the
-d parameter of
hdparm:
# hdparm -d 1 /dev/hda
/dev/hda:
setting using_dma to 1 (on)
using_dma = 1 (on)
You can do this during every boot by adding the hdparm
command to /etc/rc.d/rc.local
The -i parameter
of hdparm is also very useful, because
it gives detailed information about a disk:
# hdparm -i /dev/hda
/dev/hda:
Model=WDC WD400EB-00CPF0, FwRev=06.04G06, SerialNo=WD-WCAAT6022342
Config={ HardSect NotMFM HdSw>15uSec SpinMotCtl Fixed DTR>5Mbs FmtGapReq }
RawCHS=16383/16/63, TrkSize=57600, SectSize=600, ECCbytes=40
BuffType=DualPortCache, BuffSize=2048kB, MaxMultSect=16, MultSect=off
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=78163247
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 *udma5
AdvancedPM=no WriteCache=enabled
Drive conforms to: device does not report version:
* signifies the current active mode
In some situations it is handy to diagnose information about how memory is used. For example, on a badly performing server you may want to look whether RAM shortage is causing the system to swap pages, or maybe you are setting up a network service, and want to find the optimum caching parameters. Slackware Linux provides some tools that help you analyse how memory is used.
vmstat is a command that can provide statistics about various parts of the virtual memory system. Without any extra parameters vmstat provides a summary of some relevant statistics:
# vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
0 0 0 286804 7912 98632 0 0 198 9 1189 783 5 1 93 1
Since we are only looking at memory usage in this section, we will only have a look at the memory and swap fields.
swpd: The amount of virtual memory being used.
free: The amount of memory that is not used at the moment.
buff: The amount of memory used as buffers.
cache: The amount of memory used as cached.
si: The amount of memory that is swapped in from disk per second.
si: The amount of memory that is swapped to disk per second.
It is often useful to see how memory usage changes over time. You can add an interval as a parameter to vmstat, to run vmstat continuously, printing the current statistics. This interval is in seconds. So, if you want to get updated statistics every second, you can execute:
# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 0 0 315132 8832 99324 0 0 189 10 1185 767 5 1 93 1
1 0 0 304812 8832 99324 0 0 0 0 1222 6881 24 8 68 0
0 0 0 299948 8836 99312 0 0 0 0 1171 1824 41 9 49 0
[...]
Additionally, you can tell vmstat how many times it should output these statistics (rather than doing this infinitely). For example, if you would like to print these statistics every two seconds, and five times in total, you could execute vmstat in the following manner:
# vmstat 2 5
procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 0 0 300996 9172 99360 0 0 186 10 1184 756 5 1 93 1
0 1 0 299012 9848 99368 0 0 336 0 1293 8167 20 8 21 51
1 0 0 294788 11976 99368 0 0 1054 0 1341 12749 14 11 0 76
2 0 0 289996 13916 99368 0 0 960 176 1320 17636 22 14 0 64
2 0 0 284620 16112 99368 0 0 1086 426 1351 21217 25 18 0 58