Disk Metadata : Superblock, Directory and Inodes

Metadata

Filesystem blocks are user for 2 purposes : To store User data and Metadata

• User data - stores actual data contained in files
• Metadata - stores file system structural information such as superblock, inodes, directories

Metadata describes the structure of the file system. Most common metadata structure are superblock, inode and directories.

Superblocks

Every FS has a superblock which contains info about filesystems such as :

• File system type
• Size
• Status
• Information about other metadata structures
• • For filesystems with 1k blocksizes, a backup superblock can be found at block 8193
  • For filesystems with 2k blocksizes, at block 16384
  • For 4k blocksizes, at block 32768.

List backup superblocks:

# dumpe2fs /dev/hda3 | grep -i superblock

If Superblock  is corrupted, restore with backup :

# e2fsck -f -b 8193 /dev/sda3

 

Inode

 

 An inode is a data structure on a Linux Unix FS which stores stores basic information about a regular file, directory, or other file system objects.  

Recover deleted files used by any process in Linux from RAM memory.

Every thing in Linux is a File. A file in turn is actually a pointer to inode which contain the actual data on the disk, permissions, ownership. Now what happens when a file is deleted ? Only the link is removed by not the inode or the actual data. if a process is using the file, or if the file is open , the inode is not released for overwriting util the process is done with the file. Such files will remain in the server memory (RAM). 

Lets discuss how to recover such deleted files which is being used by a process.

Lets do it with an example.

Create a test file.
# touch testfile.txt

Echo some random data on it.
# cat /dev/random > testfile.txt

Open the file using some command like below.
# less  testfile.txt

# ps -ef | grep -i less

less 4607 root  4r  REG 254,4   21  
           8880214 /root/testing.txt (deleted)

All the open files remain in the memory and hence in the /proc filesystem. The important columns in the above output are the second one, which gives you the PID of the process that has the file open (4607), and the fourth one, which gives you the file descriptor (4). Now, we go look in /proc, where there will still be a reference to the inode, from which you can copy the file back.

# ls -l /proc/4607/fd/4
lr-x------ 1 root root 64 Apr  7 03:19 
             /proc/4607/fd/4 -> /root/testing.txt (deleted)

To recover the deleted file in memory, just copy as below.
 #cp /proc/4607/fd/4 testing.txt.bk


Hurray you got your file back. Just make sure not to use "-a" switch while copying the file as this will copy the broken softlink.

JK

 

Change Default MTA in RHEL 6 / Cent OS 6

By default Postfix is set as the default mail transfer agent in Redhat Enterprise Linux 6 and Cent OS 6 flavors.

However you can change the default Mail transfer Agent in RHEL 6 using the below commands.

* Install Sendmail if not yet installed.

# yum install sendmail -y

Change the default MTA.

# alternatives --config mta

There are 2 programs which provide 'mta'.
  Selection    Command
-----------------------------------------------
 + 1           /usr/sbin/sendmail.postfix
*  2           /usr/sbin/sendmail.sendmail
Enter to keep the current selection[+], or type selection number: 2
#

Congrats, now your default MTS is changed from postfix to Sendmail.

Regards,
Jk