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.
Tested under CHAOS4.2, RHEL5, SLES11, and FC11 (all x86_64)
Features:
Honor spa_mode() when opening the block device. Previously this
was ignored and devices were always opened read/write.
Integrated DKIOCFLUSHWRITECACHE zio operation with linux WRITE_BARRIER
for kernels post 2.6.24 where empty bio requests are supported. For
earlier kernels ENOTSUP is returned and no barriers are performed. If
RHEL5 based kernels are intended to be supported long term we may need
make use of the old akward API.
With the addition of WRITE_BARRIER support all writes which were
WRITE_SYNC can now be safely made WRITE bios. They will now take
advantage of aggregation in the elevator and improved write performance
is likely.
Notice the ZIO_FLAG_SPECULATIVE flag and pass along the hint to the
elevator by using READA instead of READ. This provides the elevator
the ability to prioritize the real READs ahead of the speculative IO
if needed.
Implement an initial version of vdev_disk_io_done() which in the case
of an EIO error triggers a media change check. If it determines a
media change has occured we fail the device and remove it from the
config. This logic I'm sure can be improved further but for now it
is an improvement over the VERIFY() that no error will ever happen.
APIs:
2.6.22 API change
Unused destroy_dirty_buffers arg removed from prototype.
2.6.24 API change
Empty write barriers are now supported and we should use them.
2.6.24 API change
Size argument dropped from bio_endio and bi_end_io, because the
bi_end_io is only called once now when the request is complete.
There is no longer any need for a size argument. This also means
that partial IO's are no longer possibe and the end_io callback
should not check bi->bi_size. Finally, the return type was updated
to void.
2.6.28 API change
open/close_bdev_excl() renamed to open/close_bdev_exclusive().
2.6.29 API change
BIO_RW_SYNC renamed to BIO_RW_SYNCIO.
The extra call to the constructor was there to reinitialize the non-
trivial primatives in the dnode (lists, mutexs, condvars, avl tree, etc).
This was safe, although not exactly clean, on Solaris because none of
the primitives allocate memory. In the Linux port this is not true.
To keep stack usage to a minimum several of the primatives dynamically
allocate memory thus initializing them twice results in a memory leak.
This patch resolves this problem for Solaris and Linux by ensuring all
*_inits are called in the constructor, and all *_destroys are called
in the destructor. Additionally we ensure that all dnode objects are
properly deconstructed before being freed to the slab, and when the
objects are allocated from the slab all required data members are
explicity initialized to correct values.
Brian Behlendorf