Sudhir Pandey's List: System administration

• The job of the primary boot loader is to find and load the secondary boot loader (stage 2). It does this by looking through the partition table for an active partition. When it finds an active partition, it scans the remaining partitions in the table to ensure that they're all inactive. When this is verified, the active partition's boot record is read from the device into RAM and executed.
• The first- and second-stage boot loaders combined are called Linux Loader (LILO) or GRand Unified Bootloader (GRUB) in the x86 PC environment. Because LILO has some disadvantages that were corrected in GRUB, let's look into GRUB. (See many additional resources on GRUB, LILO, and related topics in the Resources section later in this article.)

• If your system uses GRUB, you can change the boot runlevel by pressing the `e' key to edit the boot configuration. append the runlevel to the end of the boot command as shown:

   

   

  kernel /vmlinuz ro root=/dev/hda1 5

• The following table provides an overview of the most important parameters of standard RAID levels. Space efficiency is given as an equation in terms of the number of drives, n, which results in a value between 0 and 1, representing the fraction of the sum of the drives' capacities that is available for use. For example, if three drives are arranged in RAID 3, this gives a space efficiency of 1-(1/3) = 0.66. If their individual capacities are 250 GB each, for a total of 750 GB over the three, the usable capacity under RAID 3 for data storage is 500 GB.

• An LV can be extended by using the lvextend command. You can specify either an absolute size for the extended LV or how much additional storage you want to add to the LVM. For example:

• Add existing user tony to ftp supplementary/secondary group with usermod command using -a option ~ i.e. add the user to the supplemental group(s). Use only with -G optio

• Change the ID number for an existing user with the usermod(1M) command. You can include -U option to change the users files to the new UID, as in this example: 
   
   usermod -u 712 -U jstone

• usermod -l login-name old-name
• usermod -u UID username

• usermod -d /path/to/new/homedir/ username

• Example:
  Lock user account named vivek. Login as a root user and type following command:

  # passwd -l vivek

  Linux unlocking an account
  Syntax:
  passwd -u {username}

• A SAN alone does not provide the "file" abstraction, only block-level operations.
• However, file systems built on top of SANs do provide this abstraction, and are known as SAN filesystems or shared disk file systems
• Network-attached storage (NAS), in contrast to SAN, uses file-based protocols such as NFS or SMB/CIFS where it is clear that the storage is remote, and computers request a portion of an abstract file rather than a disk block.
• Other benefits include the ability to allow servers to boot from the SAN itself. This allows for a quick and easy replacement of faulty servers since the SAN can be reconfigured so that a replacement server can use the LUN of the faulty server.

2 more annotations...

• he term SAN can sometimes refer to system area networks instead of a storage area network. System area networks are clusters of high performance computers used for distributed processing applications requiring fast local network performance. Storage area networks, on the other, are designed specifically for data management.

• 1. The server implements NFS daemon processes (running by default as nfsd) in order to make its data generically available to clients.
   
  2. The server administrator determines what to make available, exporting the names and parameters of directories (typically using the /etc/exports configuration file and the exportfs command).
   
  3. The server security-administration ensures that it can recognize and approve validated clients.
   
  4. The server network configuration ensures that appropriate clients can negotiate with it through any firewall system.
   
  5. The client machine requests access to exported data, typically by issuing a mount command. (The client asks the server (rpcbind) which port the NFS server is using, the client connects to the NFS server (nfsd), nfsd passes the request to mountd)
   
  6. If all goes well, users on the client machine can then view and interact with mounted filesystems on the server within the parameters permitted.
   
   

  Note that automation of the NFS mounting process may take place — perhaps using /etc/fstab and/or automounting facilities.

• Default NFS server behavior is to prevent root on client machines from having privileged access to exported files. Servers do this by mapping the "root" user to some unprivileged user (usually the user "nobody") on the server side. This is known as root squashing.

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  RR	Value	RFC	Description
  A	1	RFC 1035	IPv4 Address record. An IPv4 address for a host.
  AAAA	28	RFC 3596	IPv6 Address record. An IPv6 address for a host. Current IETF recommendation for IPv6 forward-mapped zones.

• An example would look like this:  

  smbmount //servername/sharename /mountdirectory -o username=mywindowsusername,password=mywindowspassword 

    

   The mount equivelant is:  

  mount -t smbfs //servername/sharename /mountdirectory -o username=mywindowsusername,password=mywindowspassword 

    

   //servername/sharename refers to the name of the Windows computer and the name of the share.   

   /mountdirectory refers to the directory you use as the mount point on the Linux workstation. It can be any directory as long as the user executing the command has rights to it.