At last a useful user space interface for the Linux ZFS port arrives.
With the addition of the ZVOL real ZFS based block devices are available
and can be compared head to head with Linux's MD and LVM block drivers.
The Linux ZVOL has not yet had any performance work done but from a user
perspective it should be functionally complete and behave like any other
Linux block device.
The ZVOL has so far been tested using zconfig.sh on the following x86_64
based platforms: FC11, CHAOS4, RHEL5, RHEL6, and SLES11. However, more
testing is required to ensure everything is working as designed.
What follows in a somewhat detailed list of changes includes in this
commit to make ZVOL's possible. A few other issues were addressed in
the context of these changes which will also be mentioned.
* Added module/zfs/zvol.c which is based off the original Solaris ZVOL
implementation but rewritten to intergrate with the Linux block device
APIs. The basic design remains the similar in Linux with the major
change being request processing. Request processing is handled by
registering a request function which the elevator calls once all request
merges is finished and the elevator unplugs. This function is called
under a spin lock and the request structure is passed to the block driver
to be queued for IO. The elevator must be notified asyncronously once
the request completes or fails with an error. This allows us the block
driver a chance to handle many request concurrently. For the ZVOL we
maintain a taskq with a service thread per core. As requests are delivered
by the elevator each request is dispatched to the taskq. The task queue
handles each request with a write or read helper function which basically
copies the request data in to our out of the DMU object. Writes single
completion as soon as the DMU has the data unless they are marked sync.
Reads are all handled syncronously however the elevator will merge many
small reads in to a large read before it submitting the request.
* Cachine is worth specifically mentioning. Because both the Linux VFS
and the ZFS ARC both want to fully manage the cache we unfortunately
end up with two caches. This means our memory foot print is larger
than otherwise expected, and it means we have an extra copy between
the caches, but it does not impact correctness. All syncs are barrior
requests I believe are handled correctly. Longer term there is lots of
room for improvement here but it will require fairly extensive changes
to either the Linux VFS and VM layer, or additional DMU interfaces to
handle managing buffer not directly allocated by the ARC.
* Added module/zfs/include/sys/blkdev.h which contains all the Linux
compatibility foo which is required to handle changes in the Linux block
APIs from 2.6.18 thru 2.6.31 based kernels.
* The dmu_{read,write}_uio interfaces which don't make sense on Linux
have been modified to dmu_{read,write}_req functions which consume the
standard Linux IO request structure. Their function fundamentally
remains the same so this happily worked out pretty cleanly.
* The /dev/zfs character device is no longer created through the half
implemented Solaris driver DDI interfaces. It is now simply created
with it's own major number as a Linux misc device which greatly simplifies
everything. It is only capable of handling ioctls() but this fits nicely
because that's all it ever has to do. The ZVOL devices unlike in Solaris
do not leverage the same major number as /dev/zfs but instead register
their own major. Because only one major is allocated and space is reserved
for 16 partitions per-device there is a limit of 16384 concurrent ZVOL
devices. By using multiple majors like the scsi driver this limit could
be addressed if it becomes a problem.
* The {spa,zfs,zvol}_busy() functions have all be removed because they
are not required on a Linux system. Under Linux the registered module
exit function will not be called while the are still references to the
module. Once the exit function is called however it must succeed or
block, it may not fail so returning an error on module unload makes to
sense under Linux.
* With the addition of ZVOL support all the HAVE_ZVOL defines were removed
for obvious reasons. However, the HAVE_ZPL defines have been relocated
in to the linux-{kernel,user}-disk topic branches and must remain until
the ZPL is implemented.
Brian Behlendorf
Ricardo M. Correia