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solaris 上的df -k 和du -ks怎么差距这么大？？？
Why "du" and "df" report different totals of used disk space?
SHORT ANSWER
There are 3 reasons why du and df can show different answers:
1. Inconsistent fileystem requiring fsck(1m).
2. Process with open file which does not exist in filesystem.
3. Mount point directory contains data.
LONG ANSWER
Before going into detail for the 3 possibilities, it is important to
recognize how du and df obtain their answers:
. du "walks" the filesystem (like "find" command would),
checking the size of each file in turn, and keeping track of the total.
. df makes a system call to the filesystem itself and requests a
number of details, one of which is the current disk space used.
(it gets the info directly from the superblocks of the filesystem).
1. Inconsistent fileystem requiring fsck(1m).
If the filesystem becomes corrupt/inconsistent for some reason, it
is quite likely that du and df will differ. What can be seen by a
process looking at the filesystem (i.e. du), does not match up with
the view the filesystem itself has (i.e. what will be returned to
the querying df process).
Corrupt/inconsistent filesystems should be repaired using fsck(1m).
2. Process with open file which does not exist in the filesystem directory
structure.
This scenario commonly occurs when some process keeps writing to a file
(usually a logfile) and a sysadmin deletes the file in panic to prevent
the filesystem from filling up. But the offending process keeps running
and the space is not freed (the process keeps the file open).
The disk blocks associated with a file are actually deleted and made
available for reuse when the last "reference" to the file is removed.
When a Unix process opens a file, the reference count to that file is
incremented. Subsequently, if the file itself is removed from the
filesystem, the data blocks remain in use until the process closes
the file, either explicitly with close(2), or implicitly when the
process dies.
Under these conditions, du will be unable to "see" the file in the
filesystem (it was rm'd from the dir. structure), and therefore will
not count its size, but df (in getting the answer from the filesystem
itself) "knows" the file still exists.
When the process closes the file (explicitly, or implicitly when the
process either quits or is killed, or the machine is rebooted), the
disk blocks will return to the freelist and du and df will agree.
Actually it is the unmount and remount of the filesystem that fixes
this problem. But obviously if some process has an open file on the
filesystem, it will be impossible to unmount the filesystem (device busy).
3. Directory mount point containing data.
As filesystems are mounted on top of directories, if a directory
mount point contains data, the du process will be unable to see this
data (seeing only the mounted filesystem), but the underlying
filesystem will still keep track of this data, consequently df will
report the extra disk space in use.
Unmounting the filesystem will reveal the data. However, if the mounted
filesystem is being used by running processes it will not be possible
to unmount it. Either identify and kill the processes (fuser(1m), etc.),
or reboot (possibly in single user mode) to check the mount point directory. |
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发表于 2005-09-15 14:32
发表于 2005-09-15 18:01
