AIX 5L Problem Determination - IBM Redbooks

7.4 The AIX JFS
7.4.1 Creating a JFS
To synchronize the logical volume, the following command should be run:
# synclvodm -v volg01
synclvodm: Physical volume data updated.
synclvodm: Logical volume logvol01 updated.
synclvodm: Warning, lv control block of loglv00 has been over written.
0516-622 synclvodm: Warning, cannot write lv control block data.
synclvodm: Logical volume loglv00 updated.
The system can now be repaired. If the file system data was spread across all
the disks, including the failed disk, it may need to be restored from the last
backup.
Similar to the LVM, most JFS problems can be traced to problems with the
underlying physical disk.
As with volume groups, various JFS features have been added at different levels
of AIX, which preclude those file systems being mounted if the volume group was
imported on an earlier version of AIX. Such features include large file enabled file
systems, file systems with non-default allocation group size, and JFS2.
In a journaled file system (JFS), files are stored in blocks of contiguous bytes.
The default block size, also referred to as fragmentation size in AIX, is 4096
bytes (4 KB). The JFS i-node contains an information structure of the file with an
array of eight pointers to data blocks. A file that is less then 32 KB is referenced
directly from the i-node.
A larger file uses a 4-KB block, referred to as an indirect block, for the addressing
of up to 1024 data blocks. Using an indirect block, a file size of 1024 x 4 KB = 4
MB is possible.
For files larger than 4 MB, a second block, the double indirect block, is used. The
double indirect block points to 512 indirect blocks, providing the possible
addressing of 512 x 1024 x 4 KB = 2 GB files. Figure 7-2 on page 151 illustrates
the addressing using double indirection.
150 IBM ^ Certification Study Guide - AIX 5L Problem Determination Tools and Techniques