AIX 5L Problem Determination - IBM Redbooks

► Understanding enhanced journaled file system size limitations
The maximum size for an enhanced journaled file system is architecturally
limited to 4 Petabytes. I-nodes are dynamically allocated by JFS2, so you do
not need to consider how many i-nodes you may need when creating a JFS2
file system. You need to consider the size of the file system log.
► Enhanced journaled file system log size issues
In most instances, multiple journaled file systems use a common log
configured to be 4 MB in size. When file systems exceed 2 GB or when the
total amount of file system space using a single log exceeds 2 GB, the default
log size might not be sufficient. In either case, scale log sizes upward as the
file system size increases. The JFS log is limited to a maximum size of 256
MB.
► JFS2 file space allocation
File space allocation is the method by which data is apportioned physical
storage space in the operating system. The kernel allocates disk space to a
file or directory in the form of logical blocks. A logical block refers to the
division of a file or directory contents into 512, 1024, 2048, or 4096 byte units.
When a JFS2 file system is created the logical block size is specified to be
one of 512, 1024, 2048, or 4096 bytes. Logical blocks are not tangible
entities; however, the data in a logical block consumes physical storage
space on the disk. Each file or directory consists of zero or more logical
blocks.
► Full and partial logical blocks
A file or directory may contain full or partial logical blocks. A full logical block
contains 512, 1024, 2048, or 4096 bytes of data, depending on the file system
block size specified when the JFS2 file system was created. Partial logical
blocks occur when the last logical block of a file or directory contains less than
the file system block size of data.
For example, a JFS2 file system with a logical block size of 4096 with a file of
8192 bytes is two logical blocks. The first 4096 bytes reside in the first logical
block and the following 4096 bytes reside in the second logical block.
Likewise, a file of 4608 bytes consists of two logical blocks. However, the last
logical block is a partial logical block containing the last 512 bytes of the file's
data. Only the last logical block of a file can be a partial logical block.
► JFS2 file space allocation
The default block size is 4096 bytes. You can specify smaller block sizes with
the mkfs command during a file system's creation. Allowable fragment sizes
are 512, 1024, 2048, and 4096 bytes. You can use only one block’s size in a
file system.
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