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Online Tutoriale Swap Space

Posted by ascultradio on September 3, 2009

Swap Space :

General

In some cases it’s good for the swap partition to be used. For example, long running processes often access only a subset of the page frames they obtained. This means that the swap partition can safely be used even if memory is available because system memory could be better served for disk cache to improve overall system performance. In fact, in the 2.6 kernel, i.e. RHEL 4, you can define a threshold when processes should be swapped out in favor of I/O caching. This can be tuned with the /proc/sys/vm/swappiness kernel parameter. The default value of /proc/sys/vm/swappiness is 60 which means that applications and programs that have not done a lot lately can be swapped out. Higher values will provide more I/O cache and lower values will wait longer to swap out idle applications.

Depending on the system profile you may see that swap usage slowly increases with system uptime. To display swap usage you can run the free(1) command or you can check the /proc/meminfo file. When the system uses swap space it will sometimes not decrease afterward. This saves I/O if memory is needed and pages don’t have to be swapped out again when the pages are already in the swap space. However, if swap usage gets close to 80% – 100% (your threshold may be lower if you use a large swap space), then a closer look should be taken at the system, see also Checking Swap Space Size and Usage. Depending on the size of your swap space, you may want to check swap activity with vmstat or sar if swap allocation is lower than 80%. But these numbers really depend on the size of the swap space. The actual numbers of swapped pages per timeframe from vmstat or sar are the important numbers. Constant swapping should be avoided at all cost.

Note, never add a permanent swap file to the system due to the performance impact of the filesystem layer.

Swap Size Recommendations

According to Oracle9i Installation Guide Release 2 a minimum of 512MB of RAM is required to install Oracle9i Server.
According to Oracle Database Installation Guide 10g Release 2 at least 1024MB of RAM is required for 10g R2.

For 10g R2, Oracle gives the following swap space requirement:

  RAM               Swap Space
  --------------------------------------------
  1 GB - 2 GB       1.5 times the size of RAM
  2 GB - 8 GB       Equal to the size of RAM
  more than 8GB     0.75 times the size of RAM

Checking Swap Space Size and Usage

You can check the size and current usage of swap space by running one of the following two commands:

grep SwapTotal /proc/meminfo
cat /proc/swaps
free

Swap usage may slowly increase as shown above but should stop at some point. If swap usage continues to grow steadily or is already large, then one of the following choices may need to be considered:
– Add more RAM or reduce the size of the SGA
– Increase the size of the swap space

If you see constant swapping, then you need to either add more RAM or reduce the size of the SGA. Constant swapping should be avoided at all cost. You can check current swap activity using the following commands:

$ vmstat 3 100
procs                      memory      swap          io     system         cpu
 r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
 1  0      0 972488   7148  20848    0    0   856     6  138    53  0  0 99  0
 0  1      0 962204   9388  20848    0    0   747     0 4389  8859 23 24 11 41
 0  1      0 959500  10728  20848    0    0   440   313 1496  2345  4  7  0 89
 0  1      0 956912  12216  20848    0    0   496     0 2294  4224 10 13  0 77
 1  1      0 951600  15228  20848    0    0   997   264 2241  3945  6 13  0 81
 0  1      0 947860  17188  20848    0    0   647   280 2386  3985  9  9  1 80
 0  1      0 944932  19304  20848    0    0   705     0 1501  2580  4  9  0 87

The fields si and so show the amount of memory paged in from disk and paged out to disk, respectively. If the server shows continuous swap activity then more memory should be added or the SGA size should be reduced. To check the history of swap activity, you can use the sar command.
For example, to check swap activity from Oct 12th:

# ls -al /var/log/sa | grep "Oct 12"
-rw-r--r--    1 root     root      2333308 Oct 12 23:55 sa12
-rw-r--r--    1 root     root      4354749 Oct 12 23:53 sar12
# sar -W -f /var/log/sa/sa12
Linux 2.4.21-32.0.1.ELhugemem (rac01prd)       10/12/2005

12:00:00 AM  pswpin/s pswpout/s
12:05:00 AM      0.00      0.00
12:10:00 AM      0.00      0.00
12:15:00 AM      0.00      0.00
12:20:00 AM      0.00      0.00
12:25:00 AM      0.00      0.00
12:30:00 AM      0.00      0.00
...

The fields pswpin and pswpout show the total number of pages brought in and out per second, respectively.

If the server shows sporadic swap activity or swap activity for a short period time at certain invervals, then you can either add more swap space or RAM. If swap usage is already very large (don’t confuse it with constant swapping), then I would add more RAM.

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