Assessing file placement with the fileplace command

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To see how the file copied earlier, big1, is stored on the disk, we can use the fileplace command. The fileplace command displays the placement of a file's blocks within a logical volume or within one or more physical volumes.

To determine whether the fileplace command is installed and available, run the following command:

# lslpp -lI perfagent.tools

Use the following command:

# fileplace -pv big1

File: big1  Size: 3554273 bytes  Vol: /dev/hd10
Blk Size: 4096  Frag Size: 4096  Nfrags: 868   Compress: no
Inode: 19  Mode: -rwxr-xr-x  Owner: hoetzel  Group: system

  Physical Addresses (mirror copy 1)                            Logical Fragment
  ----------------------------------                            ----------------
  0001584-0001591  hdisk0     8 frags    32768 Bytes,   0.9%    0001040-0001047
  0001624-0001671  hdisk0    48 frags   196608 Bytes,   5.5%    0001080-0001127
  0001728-0002539  hdisk0   812 frags  3325952 Bytes,  93.5%    0001184-0001995

  868 frags over space of 956 frags:  space efficiency = 90.8%
  3 fragments out of 868 possible:  sequentiality = 99.8%

This example shows that there is very little fragmentation within the file, and those are small gaps. We can therefore infer that the disk arrangement of big1 is not significantly affecting its sequential read-time. Further, given that a (recently created) 3.5 MB file encounters this little fragmentation, it appears that the file system in general has not become particularly fragmented.

Occasionally, portions of a file may not be mapped to any blocks in the volume. These areas are implicitly filled with zeroes by the file system. These areas show as unallocated logical blocks. A file that has these holes will show the file size to be a larger number of bytes than it actually occupies (that is, the ls -l command will show a large size, whereas the du command will show a smaller size or the number of blocks the file really occupies on disk).

The fileplace command reads the file's list of blocks from the logical volume. If the file is new, the information may not be on disk yet. Use the sync command to flush the information. Also, the fileplace command will not display NFS remote files (unless the command runs on the server).

Note: If a file has been created by seeking to various locations and writing widely dispersed records, only the pages that contain records will take up space on disk and appear on a fileplace report. The file system does not fill in the intervening pages automatically when the file is created. However, if such a file is read sequentially (by the cp or tar commands, for example) the space between records is read as binary zeroes. Thus, the output of such a cp command can be much larger than the input file, although the data is the same.