Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Swapping is necessary for two important reasons. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. The system can swap out those pages and free the memory for other applications or even for the disk cache.
However, swapping does have a downside. Compared to memory, disks are very slow. Memory speeds can be measured in nanoseconds, while disks are measured in milliseconds, so accessing the disk can be tens of thousands times slower than accessing physical memory. The more swapping that occurs, the slower your system will be. Sometimes excessive swapping or thrashing occurs where a page is swapped out and then very soon swapped in and then swapped out again and so on. In such situations the system is struggling to find free memory and keep applications running at the same time. In this case only adding more RAM will help.
Linux has two forms of swap space: the swap partition and the swap file. The swap partition is an independent section of the hard disk used solely for swapping; no other files can reside there. The swap file is a special file in the filesystem that resides amongst your system and data files.
To see what swap space you have, use the command swapon -s. The output will look something like this:
Filename Type Size Used Priority
/dev/sda5 partition 859436 0 -1
Each line lists a separate swap space being used by the system. Here, the 'Type' field indicates that this swap space is a partition rather than a file, and from 'Filename' we see that it is on the disk sda5. The 'Size' is listed in kilobytes, and the 'Used' field tells us how many kilobytes of swap space has been used (in this case none). 'Priority' tells Linux which swap space to use first. One great thing about the Linux swapping subsystem is that if you mount two (or more) swap spaces (preferably on two different devices) with the same priority, Linux will interleave its swapping activity between them, which can greatly increase swapping performance.
To add an extra swap partition to your system, you first need to prepare it. Step one is to ensure that the partition is marked as a swap partition and step two is to make the swap filesystem. To check that the partition is marked for swap, run as root:
fdisk -l /dev/hdb
Replace /dev/hdb with the device of the hard disk on your system with the swap partition on it. You should see output that looks like this:
Device Boot Start End Blocks Id System
/dev/hdb1 2328 2434 859446 82 Linux swap / Solaris
If the partition isn't marked as swap you will need to alter it by running fdisk and using the 't' menu option. Be careful when working with partitions -- you don't want to delete important partitions by mistake or change the id of your system partition to swap by mistake. All data on a swap partition will be lost, so double-check every change you make. Also note that Solaris uses the same ID as Linux swap space for its partitions, so be careful not to kill your Solaris partitions by mistake.
Once a partition is marked as swap, you need to prepare it using the mkswap (make swap) command as root:
mkswap /dev/hdb1
If you see no errors, your swap space is ready to use. To activate it immediately, type:
swapon /dev/hdb1
You can verify that it is being used by running swapon -s. To mount the swap space automatically at boot time, you must add an entry to the /etc/fstab file, which contains a list of filesystems and swap spaces that need to be mounted at boot up. The format of each line is:
Since swap space is a special type of filesystem, many of these parameters aren't applicable. For swap space, add:
/dev/hdb1 none swap sw 0 0
where /dev/hdb1 is the swap partition. It doesn't have a specific mount point, hence none. It is of type swap with options of sw, and the last two parameters aren't used so they are entered as 0.
To check that your swap space is being automatically mounted without having to reboot, you can run the swapoff -a command (which turns off all swap spaces) and then swapon -a (which mounts all swap spaces listed in the /etc/fstab file) and then check it with swapon -s.
Swap file
As well as the swap partition, Linux also supports a swap file that you can create, prepare, and mount in a fashion similar to that of a swap partition. The advantage of swap files is that you don't need to find an empty partition or repartition a disk to add additional swap space.
To create a swap file, use the dd command to create an empty file. To create a 1GB file, type:
dd if=/dev/zero of=/swapfile bs=1024 count=1048576
/swapfile is the name of the swap file, and the count of 1048576 is the size in kilobytes (i.e. 1GB).
Prepare the swap file using mkswap just as you would a partition, but this time use the name of the swap file:
mkswap /swapfile
And similarly, mount it using the swapon command: swapon /swapfile.
The /etc/fstab entry for a swap file would look like this:
/swapfile none swap sw 0 0
How big should my swap space be?
It is possible to run a Linux system without a swap space, and the system will run well if you have a large amount of memory -- but if you run out of physical memory then the system will crash, as it has nothing else it can do, so it is advisable to have a swap space, especially since disk space is relatively cheap.
The key question is how much? Older versions of Unix-type operating systems (such as Sun OS and Ultrix) demanded a swap space of two to three times that of physical memory. Modern implementations (such as Linux) don't require that much, but they can use it if you configure it. A rule of thumb is as follows: 1) for a desktop system, use a swap space of double system memory, as it will allow you to run a large number of applications (many of which may will be idle and easily swapped), making more RAM available for the active applications; 2) for a server, have a smaller amount of swap available (say half of physical memory) so that you have some flexibility for swapping when needed, but monitor the amount of swap space used and upgrade your RAM if necessary; 3) for older desktop machines (with say only 128MB), use as much swap space as you can spare, even up to 1GB.
The Linux 2.6 kernel added a new kernel parameter called swappiness to let administrators tweak the way Linux swaps. It is a number from 0 to 100. In essence, higher values lead to more pages being swapped, and lower values lead to more applications being kept in memory, even if they are idle. Kernel maintainer Andrew Morton has said that he runs his desktop machines with a swappiness of 100, stating that "My point is that decreasing the tendency of the kernel to swap stuff out is wrong. You really don't want hundreds of megabytes of BloatyApp's untouched memory floating about in the machine. Get it out on the disk, use the memory for something useful."
One downside to Morton's idea is that if memory is swapped out too quickly then application response time drops, because when the application's window is clicked the system has to swap the application back into memory, which will make it feel slow.
The default value for swappiness is 60. You can alter it temporarily (until you next reboot) by typing as root:
echo 50 > /proc/sys/vm/swappiness
If you want to alter it permanently then you need to change the vm.swappiness parameter in the /etc/sysctl.conf file.
Conclusion
Managing swap space is an essential aspect of system administration. With good planning and proper use swapping can provide many benefits. Don't be afraid to experiment, and always monitor your system to ensure you are getting the results you need.
Thursday, June 11, 2009
Monday, April 6, 2009
How to Use a Query String in Perl
A query string is the part of a URL which is attached to the end, after the file name. It begins with a question mark and usually includes information in pairs. The format is parameter=value, as in the following example:
www.mediacollege.com/cgi-bin/myscript.cgi?topic=intro
Query strings can contain multiple sets of parameters, separated by an ampersand (&) like so:
www.mediacollege.com/cgi-bin/myscript.cgi?topic=intro&heading=1
The idea is that the information contained in the query string can be accessed and interpreted by a script. Specifically, the query string data is contained in the variable $ENV{'QUERY_STRING'}.
To Access the Query String Data
You can use a block of code like the one below to access and organise the query string:
if (length ($ENV{'QUERY_STRING'}) > 0){
$buffer = $ENV{'QUERY_STRING'};
@pairs = split(/&/, $buffer);
foreach $pair (@pairs){
($name, $value) = split(/=/, $pair);
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
$in{$name} = $value;
}
}
This code will create a hash called $in, from which you can call specific variables like so:
$topic = $in{'topic'};
$heading = $in{'heading'};
Sample code:
#!/usr/bin/perl
# Whether the method used is GET or POST, store the parameters passed in $QueryString
if($ENV{'REQUEST_METHOD'} eq "POST") {
read(STDIN, $QueryString, $ENV{'CONTENT_LENGTH'});
} else {
$type = "display_form";
$QueryString = $ENV{QUERY_STRING};
}
# split the QueryString at &'s to give you a list with each element with a format like "name=value"
@NameValuePairs = split (/&/, $QueryString);
# Split each NameValue pair, replace +'s by spaces, get the character equivalent of any data in hex (%XX) and finally, store it in an associative array
foreach $NameValue (@NameValuePairs) {
($Name, $Value) = split (/=/, $NameValue);
$Value =~ tr/+/ /;
$Value =~ s/%([\dA-Fa-f][\dA-Fa-f])/ pack ("C",hex ($1))/eg;
$in{$Name} = $Value;
$color =$in{'color'};
$name =$in{'name'};
}
print "Content-type: text/html", "\n\n";
HTML CODE HERE
www.mediacollege.com/cgi-bin/myscript.cgi?topic=intro
Query strings can contain multiple sets of parameters, separated by an ampersand (&) like so:
www.mediacollege.com/cgi-bin/myscript.cgi?topic=intro&heading=1
The idea is that the information contained in the query string can be accessed and interpreted by a script. Specifically, the query string data is contained in the variable $ENV{'QUERY_STRING'}.
To Access the Query String Data
You can use a block of code like the one below to access and organise the query string:
if (length ($ENV{'QUERY_STRING'}) > 0){
$buffer = $ENV{'QUERY_STRING'};
@pairs = split(/&/, $buffer);
foreach $pair (@pairs){
($name, $value) = split(/=/, $pair);
$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
$in{$name} = $value;
}
}
This code will create a hash called $in, from which you can call specific variables like so:
$topic = $in{'topic'};
$heading = $in{'heading'};
Sample code:
#!/usr/bin/perl
# Whether the method used is GET or POST, store the parameters passed in $QueryString
if($ENV{'REQUEST_METHOD'} eq "POST") {
read(STDIN, $QueryString, $ENV{'CONTENT_LENGTH'});
} else {
$type = "display_form";
$QueryString = $ENV{QUERY_STRING};
}
# split the QueryString at &'s to give you a list with each element with a format like "name=value"
@NameValuePairs = split (/&/, $QueryString);
# Split each NameValue pair, replace +'s by spaces, get the character equivalent of any data in hex (%XX) and finally, store it in an associative array
foreach $NameValue (@NameValuePairs) {
($Name, $Value) = split (/=/, $NameValue);
$Value =~ tr/+/ /;
$Value =~ s/%([\dA-Fa-f][\dA-Fa-f])/ pack ("C",hex ($1))/eg;
$in{$Name} = $Value;
$color =$in{'color'};
$name =$in{'name'};
}
print "Content-type: text/html", "\n\n";
HTML CODE HERE
Thursday, April 2, 2009
Linux Remote Desktop For Controlling Windows XP / Vista / Server 2003 ( rdesktop )
Connect to MS Windows 2000/2003 server from Linux, type the following command at a shell prompt (connect to Windows server called mw2sn100.mycorp.com)
$ rdesktop mw2sn100.mycorp.com
Or connect to windows XP/Vista workstation having IP 192.168.1.17:
$ rdesktop -a 24 -r sound:remote pg-svelayut &
Remote windows XP desktop
Please note that you must first enable remote desktop connection under Windows Server/XP.
* Go to Windows XP Desktop
* Right Click on My Computer
* Select properties
* Select Remote tab
* Enable Remote desktop.
* Save the changes.
Make sure enterprise firewall allows incoming connection on TCP port 3389. rdesktop supports many other options, see man page of rdesktop or visit main website of rdesktop for more information.
$ rdesktop mw2sn100.mycorp.com
Or connect to windows XP/Vista workstation having IP 192.168.1.17:
$ rdesktop -a 24 -r sound:remote pg-svelayut &
Remote windows XP desktop
Please note that you must first enable remote desktop connection under Windows Server/XP.
* Go to Windows XP Desktop
* Right Click on My Computer
* Select properties
* Select Remote tab
* Enable Remote desktop.
* Save the changes.
Make sure enterprise firewall allows incoming connection on TCP port 3389. rdesktop supports many other options, see man page of rdesktop or visit main website of rdesktop for more information.
Wednesday, April 1, 2009
Map Linux nfs drive from DOS
How to map a drive using command prompt
First click on Start and then Run. Type cmd in the Open box.
Then type the following DOS command to map the network drive:
net use x: \\computer name\share name
where x: is the drive letter you want to assign to the shared folder.
You can delete a mapped network drive using the command prompt by typing in
net use x: /delete
First click on Start and then Run. Type cmd in the Open box.
Then type the following DOS command to map the network drive:
net use x: \\computer name\share name
where x: is the drive letter you want to assign to the shared folder.
You can delete a mapped network drive using the command prompt by typing in
net use x: /delete
Friday, March 20, 2009
Suse Linux ( SLES10 ) IPv6 Configuration
How do I configure IPv6 networking under SUSE Linux Enterprise Server (SLES v10.2) or OpenSuse Linux by Novell?
IPv6 support under YaST is not upto date. You need to manually edit the configuration files. This is NOT recommended as YaST may get confused later on.
SLES10 IPv6 Configuration From The Command Line
Use the following command to set new IPv6 address using ip command under Suse Linux.
Our Sample IPv6 Setup
- IPv6 IP : 2607:f0d0:1002:0011:0000:0000:0000:0002/64
- IPv6 Default Router IP : 2607:f0d0:1002:0011:0000:0000:0000:0001
Task: Adding An IPv6 Address
Type the following command:
ip -6 address add {IPv6-Address}/{NetMask} dev {device-name}
To add 2607:f0d0:1002:0011:0000:0000:0000:0002/64 IP to eth0, enter:
# ip -6 address add 2607:f0d0:1002:0011:0000:0000:0000:0002/64 dev eth0
Task: Adding A Default Route
Type the following command:
ip -6 route add default via {IP6-Router-IP} dev {device-name}
Add a default IPv6 router IP 2607:f0d0:1002:0011:0000:0000:0000:0001, enter:
# ip -6 route add default via 2607:f0d0:1002:0011:0000:0000:0000:0001 dev eth0
Task: Display your IPv6 IP addresses configuration
Type the following command:
# ip -6 address show dev eth0
To review your IPv6 routing table, enter:
# ip -6 route show
Task: Testing your IPv6 Configuration
Type the following command:
# ping6 ipv6.google.com
# ping6 www.cyberciti.biz
Sample output:
PING6(56=40+8+8 bytes) 2607:f0d0:3001:9::2 --> 2607:f0d0:1002:11::4
16 bytes from 2607:f0d0:1002:11::4, icmp_seq=0 hlim=60 time=34.481 ms
16 bytes from 2607:f0d0:1002:11::4, icmp_seq=1 hlim=60 time=34.207 ms
16 bytes from 2607:f0d0:1002:11::4, icmp_seq=2 hlim=60 time=33.994 ms
Persistence IPv6 Configuration
Above commands will not keep IPv6 configuration across reboots unless the configuration files are updated accordingly. You need to update the following IPv6 configurations under SLES / OpenSuse Linux:
- eth0 IPv6 configuration file : /etc/sysconfig/network/ifcfg-eth-id-${ETHIDFILE}
- Default IPv6 routing configuration file : /etc/sysconfig/network/routes
You can find out the value of ETHIDFILE by typing the following command:
# ip link show dev eth0 | awk '/link/{ print $2 }'
# ETHIDFILE=$(ip link show dev eth0 | awk '/link/{ print $2 }')
Now, open configuration file, enter:
# cp /etc/sysconfig/network/ifcfg-eth-id-${ETHIDFILE} /root/ifcfg-eth-id-${ETHIDFILE}.bak
vi /etc/sysconfig/network/ifcfg-eth-id-${ETHIDFILE}
Add configuration as follows:
LABEL_0='0'
IPADDR_0='2607:f0d0:1002:0011:0000:0000:0000:0002'
PREFIXLEN_0='64'
Save and close the file. Now, update default IPv6 routing:
# cp /etc/sysconfig/network/routes /root/routes.bak
# echo 'default 2607:f0d0:1002:0011:0000:0000:0000:0001 - -' >> /etc/sysconfig/network/routes
Finally, restart the networking under SLES10:
# service network restart
Test your setup, enter:
# ping6 www.cyberciti.biz
# ping6 ipv6.google.com
Thursday, March 12, 2009
FTP Error code
# Description
110 Restart marker reply. In this case, the text is exact and not left to the particular implementation; it must read: MARK yyyy = mmmm where yyyy is User-process data stream marker, and mmmm server's equivalent marker (note the spaces between markers and "=").
120 Service ready in nnn minutes.
125 Data connection already open; transfer starting.
150 File status okay; about to open data connection.
200 Command okay.
202 Command not implemented, superfluous at this site.
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.On how to use the server or the meaning of a particular non-standard command. This reply is useful only to the human user.
215 NAME system type. Where NAME is an official system name from the list in the Assigned Numbers document.
220 Service ready for new user.
221 Service closing control connection.
225 Data connection open; no transfer in progress.
226 Closing data connection. Requested file action successful (for example, file transfer or file abort).
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
230 User logged in, proceed. Logged out if appropriate.
250 Requested file action okay, completed.
257 "PATHNAME" created.
331 User name okay, need password.
332 Need account for login.
350 Requested file action pending further information
421 Service not available, closing control connection.This may be a reply to any command if the service knows it must shut down.
425 Can't open data connection.
426 Connection closed; transfer aborted.
450 Requested file action not taken.
451 Requested action aborted. Local error in processing.
452 Requested action not taken. Insufficient storage space in system.File unavailable (e.g., file busy).
500 Syntax error, command unrecognized. This may include errors such as command line too long.
501 Syntax error in parameters or arguments.
502 Command not implemented.
503 Bad sequence of commands.
504 Command not implemented for that parameter.
530 Not logged in.
532 Need account for storing files.
550 Requested action not taken. File unavailable (e.g., file not found, no access).
551 Requested action aborted. Page type unknown.
552 Requested file action aborted. Exceeded storage allocation (for current directory or dataset).
553 Requested action not taken. File name not allowed.
110 Restart marker reply. In this case, the text is exact and not left to the particular implementation; it must read: MARK yyyy = mmmm where yyyy is User-process data stream marker, and mmmm server's equivalent marker (note the spaces between markers and "=").
120 Service ready in nnn minutes.
125 Data connection already open; transfer starting.
150 File status okay; about to open data connection.
200 Command okay.
202 Command not implemented, superfluous at this site.
211 System status, or system help reply.
212 Directory status.
213 File status.
214 Help message.On how to use the server or the meaning of a particular non-standard command. This reply is useful only to the human user.
215 NAME system type. Where NAME is an official system name from the list in the Assigned Numbers document.
220 Service ready for new user.
221 Service closing control connection.
225 Data connection open; no transfer in progress.
226 Closing data connection. Requested file action successful (for example, file transfer or file abort).
227 Entering Passive Mode (h1,h2,h3,h4,p1,p2).
230 User logged in, proceed. Logged out if appropriate.
250 Requested file action okay, completed.
257 "PATHNAME" created.
331 User name okay, need password.
332 Need account for login.
350 Requested file action pending further information
421 Service not available, closing control connection.This may be a reply to any command if the service knows it must shut down.
425 Can't open data connection.
426 Connection closed; transfer aborted.
450 Requested file action not taken.
451 Requested action aborted. Local error in processing.
452 Requested action not taken. Insufficient storage space in system.File unavailable (e.g., file busy).
500 Syntax error, command unrecognized. This may include errors such as command line too long.
501 Syntax error in parameters or arguments.
502 Command not implemented.
503 Bad sequence of commands.
504 Command not implemented for that parameter.
530 Not logged in.
532 Need account for storing files.
550 Requested action not taken. File unavailable (e.g., file not found, no access).
551 Requested action aborted. Page type unknown.
552 Requested file action aborted. Exceeded storage allocation (for current directory or dataset).
553 Requested action not taken. File name not allowed.
Thursday, March 5, 2009
FTP Commands
| ? | to request help or information about the FTP commands | |
|---|---|---|
| ascii | to set the mode of file transfer to ASCII (this is the default and transmits seven bits per character) | |
| binary | to set the mode of file transfer to binary (the binary mode transmits all eight bits per byte and thus provides less chance of a transmission error and must be used to transmit files other than ASCII files) | |
| bye | to exit the FTP environment (same as quit) | |
| cd | to change directory on the remote machine | |
| close | to terminate a connection with another computer | |
| | close brubeck | closes the current FTP connection with brubeck, but still leaves you within the FTP environment. |
| delete | to delete (remove) a file in the current remote directory (same as rm in UNIX) | |
| get | to copy one file from the remote machine to the local machine | |
| | get ABC DEF | copies file ABC in the current remote directory to (or on top of) a file named DEF in your current local directory. |
| | get ABC | copies file ABC in the current remote directory to (or on top of) a file with the same name, ABC, in your current local directory. |
| help | to request a list of all available FTP commands | |
| lcd | to change directory on your local machine (same as UNIX cd) | |
| ls | to list the names of the files in the current remote directory | |
| mkdir | to make a new directory within the current remote directory | |
| mget | to copy multiple files from the remote machine to the local machine; you are prompted for a y/n answer before transferring each file | |
| | mget * | copies all the files in the current remote directory to your current local directory, using the same filenames. Notice the use of the wild card character, *. |
| mput | to copy multiple files from the local machine to the remote machine; you are prompted for a y/n answer before transferring each file | |
| open | to open a connection with another computer | |
| | open brubeck | opens a new FTP connection with brubeck; you must enter a username and password for a brubeck account (unless it is to be an anonymous connection). |
| put | to copy one file from the local machine to the remote machine | |
| pwd | to find out the pathname of the current directory on the remote machine | |
| quit | to exit the FTP environment (same as bye) | |
| rmdir | to to remove (delete) a directory in the current remote directory | |
Wednesday, March 4, 2009
NFS Stale File Handle error and solution
Sometime NFS can result in to weird problems. For example NFS mounted directories sometimes contain stale file handles. If you run command such as ls or vi you will see an error:
$ ls
.: Stale File Handle
First let us try to understand the concept of Stale File Handle. Managing NFS and NIS, 2nd Edition book defines filehandles as follows (a good book if you would like to master NFS and NIS):
A filehandle becomes stale whenever the file or directory referenced by the handle is removed by another host, while your client still holds an active reference to the object. A typical example occurs when the current directory of a process, running on your client, is removed on the server (either by a process running on the server or on another client).
So this can occur if the directory is modified on the NFS server, but the directories modification time is not updated.
How do I fix this problem?
a) The best solution is to remount directory from the NFS client using mount command:# umount -f /mnt/local# mount -t nfs nfsserver:/path/to/share /mnt/local
First command (umount) forcefully unmount a disk partition /mnt/local (NFS).
(b) Or try to
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