Creating a RAM Disk on CentOS 6

tmpfs vs. ramfs

The two main RAM based file system types in Linux are tmpfs and ramfs. ramfs is the older file system type and is largely replaced in most scenarios by tmpfs.

ramfs
file systems cannot be limited in size like a disk base file system which is limited by it’s capacity. ramfs will continue using memory storage until the system runs out of RAM and likely crashes


tmpfs
Is a more recent RAM file system which overcomes many of the drawbacks with ramfs. You can specify a size limit in tmpfs which will give a ‘disk full’ error when the limit is reached.

There are many reasons to create a RAM Disk. One reason is to have a isolated latency test or throughput test between interconnect, but discounting the effects of the spinning disk I/O that might be the bottleneck to the test.

Step 1: Check how much RAM you have. Display it in GB

# free -g
[root@~]# free -g
             total       used       free     shared    buffers     cached
Mem:            94         92          1          0          0         67
-/+ buffers/cache:         24         69 
Swap:            1          0          1 

You can also display with -m (MB) or -k (KB)

Step 2: Create and Mount a RAM Disk

# mkdir /mnt/ramdisk
# mount -t tmpfs -o size=40g tmpfs /mnt/ramdisk

Step 3: If you wish to create automatic mount, do place it at /etc/fstab

tmpfs       /mnt/ramdisk tmpfs   nodev,nosuid,noexec,nodiratime,size=40g   0 0  

[root@ ~]# dd if=/dev/zero of=/mdsdata/ramdisk/test bs=1G count=50
dd: writing `/mdsdata/ramdisk/test': No space left on device
41+0 records in
40+0 records out
42949672960 bytes (43 GB) copied, 44.5053 s, 965 MB/s

Above if you try to create bigger file the ramdisk the it will fail. So we can limit size.

How To Modify the Sudoers File

How To Modify the Sudoers File

You will be presented with the /etc/sudoers file in your selected text editor.

/etc/sudoers

Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

root    ALL=(ALL:ALL) ALL

%admin  ALL=(ALL) ALL
%sudo   ALL=(ALL:ALL) ALL

#includedir /etc/sudoers.d

Let's take a look at what these lines do.

Code:

%admin ALL=(ALL)NOPASSWD:/usr/bin/apt-get

To break it down:

• %admin - All users of the admin group
• ALL= - from any Host/IP
• (ALL) - can run as any user
• NOPASSWD - with no password required
• :/usr/bin/apt-get - the list of comma, separated, applications.

User Privilege Lines

The fourth line, , which dictates the root user's sudo privileges, is different from the preceding lines. Let's take a look at what the different fields mean:

• root ALL=(ALL:ALL) ALL
  The first field indicates the username that the rule will apply to (root).
• demo ALL=(ALL:ALL) ALL
  The first "ALL" indicates that this rule applies to all hosts.
• demo ALL=(ALL:ALL) ALL
  This "ALL" indicates that the root user can run commands as all users.
• demo ALL=(ALL:ALL) ALL
  This "ALL" indicates that the root user can run commands as all groups.
• demo ALL=(ALL:ALL) ALL
  The last "ALL" indicates these rules apply to all commands.

This means that our root user can run any command using sudo, as long as they provide their password.

Group Privilege Lines

- Names beginning with a "%" indicate group names.

How To Set Up Custom Rules

Now that we have gotten familiar with the general syntax of the file, let's create some new rules.

How To Create Aliases

The sudoers file can be organized more easily by grouping things with various kinds of "aliases".
For instance, we can create three different groups of users, with overlapping membership:

/etc/sudoers

. . .
User_Alias      GROUPONE = abby, brent, carl
User_Alias      GROUPTWO = brent, doris, eric, 
User_Alias      GROUPTHREE = doris, felicia, grant
. . .

Group names must start with a capital letter. We can then allow members of GROUPTWO to update the apt database by creating a rule like this:

/etc/sudoers

. . .
GROUPTWO    ALL = /usr/bin/apt-get update
. . .

If we do not specify a user/group to run as, as above, sudo defaults to the root user.
We can allow members of GROUPTHREE to shutdown and reboot the machine by creating a "command alias" and using that in a rule for GROUPTHREE:

/etc/sudoers

. . .
Cmnd_Alias      POWER = /sbin/shutdown, /sbin/halt, /sbin/reboot, /sbin/restart
GROUPTHREE  ALL = POWER
. . .

We create a command alias called POWER that contains commands to power off and reboot the machine. We then allow the members of GROUPTHREE to execute these commands.
We can also create "Run as" aliases, which can replace the portion of the rule that specifies the user to execute the command as:

/etc/sudoers

. . .
Runas_Alias     WEB = www-data, apache
GROUPONE    ALL = (WEB) ALL
. . .

This will allow anyone who is a member of GROUPONE to execute commands as the www-data user or the apache user.
Just keep in mind that later rules will override earlier rules when there is a conflict between the two.

GROUPONE    ALL = NOPASSWD: /usr/bin/updatedb
. . .

NOPASSWD is a "tag" that means no password will be requested. It has a companion command called

> To find out just exactly what sudo permissions you have on your computer, you would run the following

 

# sudo -l

That is "sudo" and a lowercase L as the argument.

Code:

%admin jaunty=(ALL)NOPASSWD:/usr/bin/apt-get

So, as long as your hostname remains as 'jaunty', then you can run the sudo command from only your computer.

The second way is by using IP addresses.
Note: This requires for you to be connected to a network! Else you will be denied access

Code:

%admin 192.168.1.0/255.255.255.0=(ALL)NOPASSWD:/usr/bin/apt-get

192.168.1.0 is the IP of your local network.
255.255.255.0 is the subnet of your local network.

So, in this instance, you are restricting the use of sudo only to users with an IP address of 192.168.1.1 through to 192.168.1.254

Restricting User Switching
To run commands as another user (other than root), you would run the following:

sudo -u username command

To prevent this from happening, you can restrict it by using:

Code:

%admin jaunty=(root)NOPASSWD:/usr/bin/apt-get

So now members of the admin group can only run the given commands without a password as root. Doesn't matter how hard they try otherwise.

Note: if a previous permission is set so the user can run the command as any user.
More specifically this line that is the default in Ubuntu:

Code:

%admin ALL=(ALL) ALL

Then they will have to provide their own password to continue.

A more secure method:

Code:

%admin ALL=(root) ALL

Where they will be instead denied if they try to run an application as another user.

Restricting Application Usage
As well, as limiting the applications a user can run using sudo, you can limit the arguments of those applications that the user can use also, for apps that do more than one job.

To limit apt-get usage to just 'update' and 'upgrade', we can have something like this:

Code:

%admin jaunty=(root)NOPASSWD:/usr/bin/apt-get update,/usr/bin/apt-get upgrade

And now everything other apt-get argument (install, remove, dist-upgrade) is denied!

Alternately, we can also use the glob match '*'.

Code:

%admin jaunty=(root)NOPASSWD:/usr/bin/apt-get up*

The '*' match is very powerful, and can apply to anything in the command listing part of the configuration line, ie:

Code:

%admin jaunty=(root)NOPASSWD:/*/sbin/*

This could match anything from all the files in '/usr/sbin/' to '/usr/local/sbin/' and even places such as '/home/user/sbin/' fall into the match. As such, it is advised that you use it wisely.

Restricting use of sudo to Single Users
As well as specifying groups, we can hand out sudo permissions on a 'per-user' basis too. For example, to only allow the user 'iain' (me) to run 'apt-get update' and 'apt-get upgrade', we use the following:

Code:

iain jaunty=(root)NOPASSWD:/usr/bin/apt-get update,/usr/bin/apt-get upgrade

Note: The difference between a user and a group. Groups have the '%' symbol prefixed against their name. So, to change this so only users in the group 'iain' can run 'apt-get', we simply add the '%' symbol:

Code:

%iain jaunty=(root)NOPASSWD:/usr/bin/apt-get update,/usr/bin/apt-get upgrade

Restrict Applications
There is a hidden 5th that I forgot to mention. In some cases, you want to restrict the application once it has been given root powers.

ie: vim, less and some other similar applications can allow you to 'shell out' of the application and run commands using the ! operator, as an example:

Code:

:!aptitude

The risk? If given the permissions to running vim as root, you can carry out any administrative task on the system. Which isn't very good if you just want certain users to run vim as root, but not any other command.

For this, we have the NOEXEC option, with will prevent the command from shelling out.

Code:

%admin ALL=(root)NOEXEC:/usr/bin/vim

Although, bare in mind that for the majority of applications, this isn't the best option for usability, since programs such as 'apt-get' do indeed fork a shell to run applications such as dpkg and wget.

Host Alias Specification
Host aliases are declared as so, to use my hostname as an example:

Code:

Host_Alias HOST = jaunty

If I were to alias the local network:

Code:

Host_Alias LAN = 192.168.1.0/255.255.255.0

User Alias Specification

Code:

User_Alias FUSE_USERS = andy,ellz,matt,jamie

RunAs Alias Specification
The runas alias, are aliases for the users you can sudo as, via the 'sudo -u' command. Again, I won't go into this, but it works like so:

Code:

Runas_Alias USERS = root,andy,ellz,matt,jamie

And put in the following context:

Code:

%admin  ALL=(USERS) ALL

Members of the admin group can run any command as any of the users enlisted in USERS.

Command Alias Specification
And lastly, the command alias, as you may have guessed, are aliases for the command names. To skip any brief talk about them, lets first fufil what my original scenario intended.

Code:

Cmnd_Alias CRYPT   = /usr/bin/truecrypt
Cmnd_Alias USBDEV  = /usr/bin/unetbootin,/usr/bin/gnome-format
Cmnd_Alias APT     = /usr/bin/apt-get update,/usr/bin/apt-get upgrade
Cmnd_Alias UPDATES = /usr/bin/update-manager
Cmnd_Alias FUSE    = /usr/bin/Gmount-iso
Cmnd_Alias MYPROGS = CRYPT,USBDEV,APT,UPDATES,FUSE

Woah, that is alot. Infact, what I've done is split the applications into sub-groups, and shuffled those groups into one, MYPROGS.

Gelling it together
With the above in place, we can now write out lines such as this:

Code:

iain HOME=(root)NOPASSWD:MYPROGS

Which result in a much cleaner, easier to maintain configuration.

The Result
Put that all together, and we have something that looks like this:

Code:

Defaults    env_reset,tty_tickets

# Host alias specification
Host_Alias HOST = jaunty
Host_Alias LAN  = 192.168.1.0/255.255.255.0
Host_Alias HOME = HOST,LAN

# User alias specification

# Cmnd alias specification
Cmnd_Alias CRYPT   = /usr/bin/truecrypt
Cmnd_Alias USBDEV  = /usr/bin/unetbootin,/usr/bin/gnome-format
Cmnd_Alias APT     = /usr/bin/apt-get update,/usr/bin/apt-get upgrade
Cmnd_Alias UPDATES = /usr/bin/update-manager
Cmnd_Alias FUSE    = /usr/bin/Gmount-iso
Cmnd_Alias MYPROGS = CRYPT,USBDEV,APT,UPDATES,FUSE

# User privilege specification
root    ALL=(ALL) ALL

# Members of the admin group may gain root privileges
%admin HOME=(root) ALL
%admin HOME=(root) NOEXEC:/usr/bin/vim
iain   HOME=(root) NOPASSWD:MYPROGS

 

Find Files Based on Access / Modification / Change Time

The basic syntax of the find command is:

find path options

where path is the path to search and options are the options given to find command.

You can find files based on following three file time attribute.

• -amin  when the file was accessed in minutes
• -atime when the file was accessed in days
• -cmin when the file was created in minutes
• -ctime when the file was created in days
• -mmin when the file was modified in minutes

In all our below examples, the path is our current directory and hence we use .(dot).

1. To find files modified in the last 5 days:
find . -mtime -5  
2. To find files modified before 5 days:
find . -mtime +5

 Note: Developers, be aware. + is not default in find. If you omit the '+', it has a different meaning. It means to find files modified exactly before 5 days.

3. To find files modified in the last 40mins:
find . -mmin -40 

4. To find files modified before 40mins:
find . -mmin +40

 

The above commands will find both files and directories modifying the criteria. 

 

If you want to find only files, use the -type option.

find . -type f -mmin -40

 

This will find only the files modified in the last 40 mins, not directories.

 

5. Find files whose content got updated within last 1 hour

# find . -mmin -60
In the same way, following example finds all the files (under root file system /) that got updated within the last 24 hours (1 day).

# find / -mtime -1
Example 2: Find files which got accessed before 1 hour
# find -amin -60
 
In the same way, following example finds all the files (under root file system /) that got accessed within the last 24 hours (1 day).

# find / -atime -1
Example 3: Find files which got changed exactly before 1 hour
# find . -cmin -60
 
In the same way, following example finds all the files (under root file system /) that got changed within the last 24 hours (1 day).

# find / -ctime -1
Example 9: ls -l in find command output. Long list the files which are edited within the last 1 hour.
# find -mmin -60

How to scan new FC LUNS and SCSI disks in Linux ?

HOW TO ARTICLES- LINUX

How to scan new FC LUNS and  SCSI disks in Redhat Linux without rebooting the server?  Most of the Linux beginners have wondering how to do this and this article will be for them.It may look like very simple as we perform this in daily operation to scan luns but system has many work to do in background when you execute storage scanning commands. Redhat says this type of scan can be distributive,since it can cause delays while I/O operation timeout and remove devices unexpectedly from OS.So perform this scan when really you want to scan the disks and LUNS.

Scanning FC-LUN’s in Redhat Linux

1.First find out how many disks are visible in “fdisk -l” .

# fdisk -l 2>/dev/null | egrep '^Disk' | egrep -v 'dm-' | wc -l

2.Find out how many host bus adapter configured in the Linux box.you can use “systool -fc_host -v” to verify available FC in the system.

# ls /sys/class/fc_host
host0  host1

In this case,you need to scan host0 & host1 HBA.


3.If the system virtual memory is too low ,then do not proceed further.If you have enough free virtual memory,then you can proceed with below command to scan new LUNS.

# echo "1" > /sys/class/fc_host/host0/issue_lip
# echo "- - -" > /sys/class/scsi_host/host0/scan
# echo "1" > /sys/class/fc_host/host1/issue_lip
# echo "- - -" > /sys/class/scsi_host/host1/scan

Note: You need to monitor the “issue_lip” in /var/log/messages to determine when the scan will complete.This operation is an asynchronous operation.

You can also use rescan-scsi-bus.sh script to detect new LUNS.

# yum install sg3_utils
# ./rescan-scsi-bus.sh

4.Verify if the new LUN is visible or not by counting the available disks.

# fdisk -l 2>/dev/null | egrep '^Disk' | egrep -v 'dm-' | wc -l

If any new LUNS added ,then you can see more count is more then before scanning the LUNS.

Scanning SCSI DISKS in Redhat Linux

1.Finding the existing disk from fdisk.

[root@mylinz1 ~]# fdisk -l |egrep '^Disk' |egrep -v 'dm-'
Disk /dev/sda: 21.5 GB, 21474836480 bytes

2.Find out how many SCSI controller configured.

[root@mylinz1 ~]# ls /sys/class/scsi_host/host
host0 host1 host2

In this case,you need to scan host0,host1 & host2.

3.Scan the SCSI disks using below command.

[root@mylinz1 ~]# echo "- - -" > /sys/class/scsi_host/host0/scan
[root@mylinz1 ~]# echo "- - -" > /sys/class/scsi_host/host1/scan
[root@mylinz1 ~]# echo "- - -" > /sys/class/scsi_host/host2/scan

4.Verify if the new disks are visible or not.

[root@mylinz1 ~]# fdisk -l |egrep '^Disk' |egrep -v 'dm-'
Disk /dev/sda: 21.5 GB, 21474836480 bytes
Disk /dev/sdb: 1073 MB, 1073741824 bytes
Disk /dev/sdc: 1073 MB, 1073741824 bytes

From Redhat Linux 5.4 onwards, redhat introduced “/usr/bin/rescan-scsi-bus.sh” script to scan all the SCSI bus and update the SCSI layer to reflect new devices.
But most of the time,script will not be able to scan new disks and you need go with echo command.
Do not forget to check out Redhat Enterprise Linux 7 Tutorial .

Basic NFS Configuration

1. Basic NFS Configuration

1.1. NFS Server configuration

Run the below commands to begin the NFS Server installation:

[nfs-server ]# yum install nfs-utils rpcbind

Next we export some arbitrary directory called /opt/nfs. Create /opt/nfs directory:

[nfs-server ]# mkdir -p /opt/nfs

and edit /etc/exports NFS exports file to add the below line while replacing the IP address with the IP address of your client:

/opt/nfs NFS_client_IP(no_root_squash,rw,sync)

Next make sure to enable 2049 port on your firewall to allow clients requests:

[nfs-server ]# firewall-cmd --zone=public --add-port=2049/tcp --permanent
[nfs-server ]# firewall-cmd --reload

Start rpcbind daemon and NFS server in this order:

[nfs-server ]# service rpcbind start; service nfs start

Check the NFS server status:

[nfs-server ]# service nfs status 

1.2. NFS Client configuration

To be able to mount NFS exported directories on your client the following packages needs to be installed. Depending on your client's Linux distribution the installation procedure may be different. On Redhat 7 Linux the installation steps are as follows:

[nfs-client ]# yum install nfs-utils rpcbind
[nfs-client ]# service rpcbind start

What remains is to create a mount point directory eg. /mnt/nfs and mount previously NFS exported /opt/nfs directory:

[nfs-client ]# mkdir -p /mnt/nfs
[nfs-client ]# mount NFS-Server_IP:/opt/nfs /mnt/nfs/

Test correctness of our setup between NFS Server and NFS client. Create an arbitrary file within NFS mounted directory on the client side:

[nfs-client ]# cd /mnt/nfs/
[nfs-client ]# touch NFS.test
[nfs-client ]# ls -l
total 0
-rw-r--r--. 1 root root 0 Dec 11 08:13 NFS.test

Move the the server side and check whether our newly NFS.test file exists:

[nfs-server ]# cd /opt/nfs/
[nfs-server ]# ls -l
total 0
-rw-r--r--. 1 root root 0 Dec 11 08:13 NFS.test

2. Configuring permanent NFS mount

In order to have our NFS exports permanently available after the NFS server system reboot we need to make sure that nfs service starts after reboot:

[nfs-server ]# systemctl enable nfs-server
ln -s '/usr/lib/systemd/system/nfs-server.service' '/etc/systemd/system/nfs.target.wants/nfs-server.service'

To allow client to mount NFS exported directory permanently after reboot we need to define a mount procedure within /etc/fstab config file. Open /etc/fstab file and add the following line:

NFS-Server_IP:/opt/nfs /mnt/nfs nfs defaults   0 0

RHEL 7: Install and setup samba server

RHEL 7 : Install and setup samba server ( file sharing)

Operating System : RHEL7/CentOS 7
Arch : x86_64
Samba Version : Samba 4.1.1 (Default from CentOS repo)
Kernel Versio : 3.10.0-123.el7.x86_64
IP Address : 192.168.56.102
Network Subnet : 192.168.56.0/24 or can be written also as 192.168.56.0/255.255.255.0
NOTE: CentOS 7 is installed with minimal packages (minimal installed)

Samba Server port numbers

137/tcp
137/udp
138/tcp
138/udp
139/udp
139/udp
445/tcp
445/udp

Daemon used in Samba server

Samba services are implemented as two daemons:
1. smbd, It provides the file and printer sharing services.
2. nmbd It provides the NetBIOS-to-IP-address name service. NetBIOS over TCP/IP requires some method for mapping NetBIOS computer names to the IP addresses of a TCP/IP network.

How to install and configure samba server

Step 1 : Use yum command to install samba packages
(a) policycoreutils-python = For semanage command
(b) samba-client : For smbpasswd
(c) cups-libs : For printer service
(d) samba and samba-commons : For Samba server

yum install -y samba samba-commons cups-libs policycoreutils-python samba-client

Step 2: create a directory
Create a directory called sharedrepo in / (main root) . This directory will be shared with clients.

mkdir /home/smbshare

Step 3: Add a new group or can use existing group
To provide access on shared directory,Here we are adding new group called staff.

groupadd smb

Step 4: Change the group and permission of sharing folder
Here we are using /sharedrepo in samba server, hence group and permission are changing for this directory.

chgrp -R smb /sharedrepo
chmod -R 777 /sharedrepo

Step 5: Change the selinux security context
Change the selinux security context on sharing directory and set the selinux boolean value for samba.
You can skip this step in case you disable selinux on system.

# semanage fcontext -a -t samba_share_t /home/smbshare/

# restorecon -Rv /home/smbshare/

# setsebool -P samba_enable_home_dirs on

Step 6: create user, add into group and set samba password
create user and add them in group called staff. And set the samba password for this user.

# useradd smbuser
# usermod -aG smb smbuser

# smbpasswd -a smbuser

New SMB password:
Retype new SMB password:
Added user smbuser.

Step 7: Edit /etc/samba/smb.conf file
First take backup of /etc/samba/smb.conf file then edit the smb.conf file.

cd /etc/samba/
cp -p smb.conf smb.conf.orig

And add the below given contents in last line of /etc/samba/smb.conf file.

vi /etc/samba/smb.conf

[smbshare]
comment = shared-directory
path = /home/smbshare
public = no
valid users = sbmuser, @smb
writable = yes
browseable = yes
create mask = 0765

Step 8: Now start the smb and nmb services.

systemctl start smb.service
systemctl start nmb.service

Step 9 : Enable smb and nmb service at booting of system

systemctl enable smb.service
systemctl enable nmb.service

Step 10 : Add firewalld rule to allow samba

# firewall-cmd --add-service=samba --permanent

Now reload firewalld

# firewall-cmd --reload 

Note: firewalld service must be runnig for above procedure. To start firewalld use systemd command i.e systemctl start firewalld.service

How to connect to Samba Server

1. Windows :
In Windows Operatig System, open the run by pressing in combination of Start key + r. Then type in this format \\ip-address-of-samba-server\shared-

Direcory-name

\\192.168.56.102\sharedrepo

Give username and password when it will ask.

2. Linux :
smbclient must be installed on system .

(A) List the shared files or directory available in samba server

smbclient -L \\192.168.56.102 -U smbuser 

In above command,
-L = For listing shared objects.
Samba Server IP Address = 192.168.56.102
User Name =smbuser

Below given is sample output

sharad@linuxworld:~$ smbclient -L \\192.168.56.102 -U smbuser
Enter test's password: 
Domain=[MYGROUP] OS=[Unix] Server=[Samba 4.1.1]

 Sharename       Type      Comment
 ---------       ----      -------
 IPC$            IPC       IPC Service (Samba Server Version 4.1.1)
 sharedrepo      Disk      shared-directory
 test            Disk      Home Directories
Domain=[MYGROUP] OS=[Unix] Server=[Samba 4.1.1]

 Server               Comment
 ---------            -------
 LOCALHOST            Samba Server Version 4.1.1

 Workgroup            Master
 ---------            -------
 MYGROUP              LOCALHOST
sharad@linuxworld:~$

(B) Access using smb console

smbclient //192.168.56.102/sharedrepo -U smbuser

After login, you will get smb console. You can use get and put command for getting/putting the file. There are other commands you can use also.
Sample output

sharad@linuxworld:~$ smbclient //192.168.56.102/sharedrepo -U smbuser
Enter test's password: 
Domain=[MYGROUP] OS=[Unix] Server=[Samba 4.1.1]
smb: \> 
smb: \> help
?              allinfo        altname        archive        backup         
blocksize      cancel         case_sensitive cd             chmod          
chown          close          del            dir            du             
echo           exit           get            getfacl        geteas         
hardlink       help           history        iosize         lcd            
link           lock           lowercase      ls             l              
mask           md             mget           mkdir          more           
mput           newer          notify         open           posix          
posix_encrypt  posix_open     posix_mkdir    posix_rmdir    posix_unlink   
print          prompt         put            pwd            q              
queue          quit           readlink       rd             recurse        
reget          rename         reput          rm             rmdir          
showacls       setea          setmode        stat           symlink        
tar            tarmode        timeout        translate      unlock         
volume         vuid           wdel           logon          listconnect    
showconnect    tcon           tdis           tid            logoff         
..             !              
smb: \> 

(C) Mount the samba shared directory
Mount the samba shared directory in machine , your system must support cifs file system

mount -t cifs //192.168.56.102/sharedrepo -o username=test /mnt/

In Ubuntu, Click on dash home and simply access the samba server

smb://192.168.56.102/

See the sample screenshot of dash home.

Dynamically detecting new SAN disks in Linux

Dynamically detecting new SAN disks in Linux

When you have new LUNs created on the SAN fabric, zoned & mapped it to the server; how can you detect the luns on the linux server online, without rebooting it?.

When you dynamically add new disks to a Linux VM running on ESX server, how do you detect that disks on the Linux virtual machine?.

Here are the steps to do that :

1. Install sg3_utils and lsscsi package. [root@fedora01 ~]# # yum install –y sg3_utils lsscsi
   
2. The “lsscsi” command will list the disks attached to it. If you have just attached a disk, you will not be able to see it. You can also list this using “fdisk –l”
   [root@fedora01 ~]# lsscsi
   [0:0:0:0]    disk    VMware   Virtual disk     1.0   /dev/sda
   [root@fedora01 ~]#
   As you can see above, I currently have one disk connected to the system. To scan for a new device I just added, we should run rescan-scsi-bus.sh from the host.
   
3. Run the command “/usr/bin/rescan-scsi-bus.sh” , to dynamically detect and activate the new disk.
4.  
[root@fedora01 ~]# /usr/bin/rescan-scsi-bus.sh -l
Host adapter 0 (mptspi) found.
Scanning SCSI subsystem for new devices
Scanning host 0 for  SCSI target IDs  0 1 2 3 4 5 6 7, LUNs  0 1 2 3 4 5 6 7
Scanning for device 0 0 0 0 …
OLD: Host: scsi0 Channel: 00 Id: 00 Lun: 00
      Vendor: VMware   Model: Virtual disk   Rev: 1.0
      Type:   Direct-Access                  ANSI SCSI revision: 02
Scanning for device 0 0 1 0 …
NEW: Host: scsi0 Channel: 00 Id: 01 Lun: 00
      Vendor: VMware   Model: Virtual disk   Rev: 1.0
      Type:   Direct-Access                  ANSI SCSI revision: 02
1 new device(s) found.
0 device(s) removed.
[root@fedora01 ~]#

[root@fedora01 ~]# lsscsi
[0:0:0:0]    disk    VMware   Virtual disk     1.0   /dev/sda
[0:0:1:0]    disk    VMware   Virtual disk     1.0   /dev/sdb
[root@fedora01 ~]#

You see the new disk is visible. Now you can create a partition or filesystem on it.

After running those commands, check dmesg and /var/log/messages to see if there are any device detections. You can also do “fdisk -l” or “cat /proc/scsi/scsi” to see the attached LUNs. This works fine in RHEL5/6, SuSE 10, CentOS5, OEL5.