Less Known Solaris features...

So: How do you ensure, that the content of a filesystem doesn´t change while you work with the disk. ufs has a an interesting feature. It´s called lockfs you can lock the filesystem. You can lock it to an extend that you can only unmount and remount it to gather access to the data, but you can lock only some ways to access it.

Types of Locks

The lockfs command can establish a number of locks on an UFS filesystem:

• Delete Lock (-d):suspends access that could remove directory entries
• Hard Lock (-h):suspends all access to the filesystem. It can´t be unlocked. You have to unmount and remount it
• Name Lock (-n):suspends all access to the filesystem, that could remove or change directory entries
• Error Lock (-n):This lock is normally established, when UFS detects an internal inconsistency. It´s released by the usage of fsck. It suspends all access to the filesystem.
• write lock (-w):This lock suspends all accesses that could modify the filesystem
• flush log (-f): This isn´t really a lock. This option forces a synchronous flush of the named filesystem. It returns wenn all data has been written to disk and the log has been rolled

Write Lock

Okay, let´s assume we´ve mounted an UFS filesystem at /mnt.

# echo "test" > testfile1
# ls
lost+found testfile1

No problem. Our testfile found it´s way into file system. Okay, now we establish a write lock on our file system.

# lockfs -w /mnt

You set the locks with the lockfs command, the switch -w tells command to set an write lock. With a write lock, you can read a filesystem, but you can´t write to it. Okay, let´s check the existing locks. You use the lockfs command without any further options.

# lockfs   
Filesystem           Locktype   Comment/mnt                 write

When we try to add an addtional file, the write system call simply blocks.

# echo "test" > testfile2
^Cbash: testfile2: Interrupted system call

We have to break the echo command with CTRL-C. Okay, now let´s release the lock.

# lockfs -u /mnt

The -u commands lockfs to release the lock. When you list the existing locks, the lock on /mnt is gone.

# lockfs

And just to check it, we try to write "test" to testfile2 again.

# echo "test" > testfile2

Not problem. The command returns an an instance. When you check the filesystem, you will see both files.

# ls -l
total 20
drwx------ 2 root root 8192 Apr 25 18:10 lost+found
-rw-r--r-- 1 root root 5 Apr 27 03:49 testfile1
-rw-r--r-- 1 root root 5 Apr 27 03:50 testfile2

Delete lock

A rather strange kind of lock is the delete lock. It blocks all delete operations. It´s of less practical use, as you would think at first. You can´t delete a file, but you can zero it or fill it with other content. Okay, let´s use the testfile from our last example. At first we try to delete the first file:

# rm testfile1

No problem. Now we establish the delete lock. This time we add a comment. You can use this command to tell other admins why you have established the lock.

# lockfs -c "no deletes today" -d /mnt

When you check for existing locks, you will see the delete lock on /mnt and the comment:

# lockfs
Filesystem           Locktype   Comment
/mnt                 delete     no deletes today

When you try to delete the file, the rm just blocks and you have to break it with CTRL-C again:

# rm testfile2
^C

When you´ve delete-locked an filesystem, you can create new files, you can append data to ist, you can overwrite it:

# echo "test" > testfile3
# echo "test" >> testfile3
# echo "test" > testfile3

There is only one thing you can´t do with this new file: You simply can´t delete it.

# rm testfile3
^C

Now we release the lock

# lockfs -u /mnt

Now you can clean up your testdirectory /mnt again.

# rm testfile2
# rm testfile3

Conclusion

The lockfs is a really neat feature to deny certain accesses to your filesystem without unmounting it completly. Some locks are more useful for general use than other other. For example the write lock is really useful, when you want to freeze the content of the filesystem while working with tools like AVS. Delete lock or name lock are more of use, when you need a stable directory. The usecase is more an internal one for other tools.

Do you want to learn more?

Documentation
docs.sun.com: man page lockfs