Tag Archives: systems

Journaled file systems structure

 

 

Superblock : File System size and indentification, Free list, fragment size, nbpi.

Inodes: File size, ownership, permissions, pointers to data blocks.

Blocks: Data blocks contain data and Indirect block contain pointers to data blocks.

Journaled file systems are built within logical volumes. Because journaled file systems exist within logical volumes, the size of the file system always multiples of the logical partition size for that logical volume (for example, 4 MB). An individual file within a file system will by default have units allocated to it in blocks of  4096 bytes. (This may change if you have implemented fragmentation or large files – to be discussed later.)
Some unix commands often report file sizes in units of 512 bytes to remain compatible with other UNIX file systems. This is independent of the actual unit of allocation. The first addressable logical block on the file system is the superblock. The superblock contains information such as the file system name, size, number of inodes, date/time of creation.
The superblock is critical to the file system and if corrupted, prevents the file system from mounting. For this reason a backup copy of the superblock is always written in block 31. Uempty Immediately following the superblock are inodes which contain identifying information for files such as the file type, size, permissions, user/group/owner, create/modification and last
access dates. They also contain pointers to the data block for fragment addresses which hold the data. For larger files the system creates sets of indirect blocks filled with data block addresses to point to the data block or fragments which hold the data.

Compaq Tru64 UNIX: Setting up Standard File Systems

The following table shows the variables and their corresponding values.


Variables in Standard File Systems

 
Variable Description
<diskname> Full path of a disk, for example, /dev/rz11c
<rawdiskname> Full path of a raw disk, for example, /dev/rrz11c
<disktype> Disk type, for example, rz28
<mountpointname> Full path of a mount point, for example,
SAP DB: /sapdb/<SAPSID>/sapdata1
Informix: Not applicable
Oracle: /oracle/<SAPSID>/sapdata1


To set up a standard file system:

1. Ask the customer which disks are available and make sure that they do not belong to
one of the categories listed in the following table:


Avoidable Disk Categories

Disk Category How to Check
Mounted disks grep <diskname> /etc/fstab
Swapdevices swapon -s | grep <diskname>
Advanced FS cd /etc/fdmns; showfdmn * | grep <diskname>
Volumes within LSM (Logical Storage manager) volprint -Ath | grep <diskname>


2. Create disk label, if necessary:
disklabel -w -r <rawdiskname> <disktype>
3. Create empty file system:
newfs -m 0 <rawdiskname>

4. Create mount point:
mkdir <mountpointname>
5. Add line to /etc/fstab:
<diskname> <mountpointname> ufs
6. Mount file system:
mount <mountpointname>