Slow speed for UFS mounted drive in Linux

I just recently ran into this problem and solved it by rebooting but after further investigation it appears that issuing the following command might fix it.

echo running > /sys/block/device-name/device/state

I think you might want to look at look at section 25.14.4: Changing the Read/Write State of an Online Logical Unit in this document, however, I recommend rebooting.

Alright... Let's attempt an answer to this question.

The setup and master/skeleton directories

The "root" directory of the jail system is /home/j. It contains 3+ subdirectories:

/home/j/
   |- mroot/       # The master (read-only) root directory
   |- skel/        # The master (read-write) directory
   |- js/          # Each jail will have a subdirectory here
                   # where its writable directories point to
   |- <jail>/      # Each jail will have its own directory

Now the mroot/ directory contains the base system as you've done a make installword to it. It will keep the read-only directories such as bin/ and dev/. All writable directories (e.g. etc/) will be moved to the skel/ directory. This is done in step 3 of section 14.5.1.

As of now, there is no /home/j/mroot/etc/ folder (since you moved it), so we'll create a symlink for it. First we'll create a new subdirectory s/ (short names can be too short and thus become confusing too easily). This is done in step 5. The setup of the mroot/ folder will now look a bit like this.

/home/j/mroot/
   |- s/             # Empty directory for writing to
   |- etc            # Symlink to s/etc/
   |- home           # Symlink to s/home/
   |- ...            # More symlinks are created

These symlinks will now be broken as those directories do not exist. They will be created when a jail is created.

Creating a jail

Now the third folder (/home/j/js/) will come into play. In here we will create a subdirectory for each jail to contain the writable content.

/home/j/js/
   |- mail/
   |- ns/
   |- www/

The skeleton directory (skel/) is copied to each of these subdirectories.

Using nullfs mounts we'll now tie everything together. Each jail gets another directory. These are nullfs (read-only) mounts to /home/j/mroot (step 1 of 15.5.2).

/home/j/
   |- mail/
   |- ns/
   |- www/

As they are mounts, going into such a directory will bring you to /home/j/mroot/ instead, though your path won't change. Each jail will have a writable /etc/ folder which seems to be /home/j/mail/etc/ but due to the above created mount, it will actually be /home/j/mroot/etc/.

However, remember from the very beginning that this folder doesn't exist anymore. It was moved and instead a symlink was created. This symlink pointed to a not-yet-existing subfolder of /home/j/mroot/s/. The second nullfs mount will fix this.

When creating a mount for /home/j/ns/s to point to /home/js/ns. This destination contains the copy of the skeleton directory containing all writable files. The source, /home/j/ns/s, will actually be /home/j/mroot/s due to the first mount.

Example

When a jail accesses a file in /etc/ (which should be writable), it is actually accessing /home/j/<jail>/etc/ on the host. However, this is a mount to /home/j/mroot/etc/ due to the first mount. But this "directory" is not a directory at all, it's a symlink to /home/j/mroot/s/etc which is, due to the second mount, in fact /home/js/ns/etc/.

Conclusion

I hope I succeeded in clarifying the setup a bit. I will reread my own answer once I've cleared my mind again because my head is still spinning from trying to figure this one out myself!