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.
* zvol_create_link_common() simplified to simply issue to ioctl to
create the device and then wait up to 10 seconds for it to appear.
The device will be created within a few miliseconds by udev under
/dev/<pool>/<volume>. Note this naming convention is slightly
different than on Solaris by I feel is more Linuxy.
* Removed support for dump vdevs. This concept is specific to Solaris
and done not map cleanly to Linux. Under Linux generating system cores
is perferably done over the network via netdump, or alternately to a
block device via O_DIRECT.
When creating partition tables we always need to wait until not
only the /dev/<disk><part> device appears. But just as importantly
if we were originally given a udev path we need to wait for the
/dev/disk/*/<name>-part<part> symlink to be created. However,
since the partition naming convention differs between /dev/ and
/dev/disk we determine based on the path which convention to
expect and then wait (for a few seconds) for the device to be
created. Based on my experience with udev on my test nodes it
takes about 300ms for the devices to be created after being
prompted by the kernel. This time will vary somehwat based
on how complicated your udev rules are, so for safety I threw
in a factor of 10. We wait 3 seconds for the devices to appears
before erroring out with a failure.
An additional minor fix includes checking the force flag in the
EFI_GPT_PRIMARY_CORRUPT case. This allows you to force the
update even in the corrupt partition case.
Finally, since these are Linux only changes I've dropped the
devid code entirely here because I still can't think of why we
would need or want it on a Linux system.
The major change here is to fix up libefi to be linux aware. For
the most part this wasn't too hard but there were a few major issues.
First off I needed to handle the DKIOCGMEDIAINFO and DKIOCINFO ioctls.
There is no direct equivilant for these ioctls under linux. To handle
this I added wrapper functions which under Solaris simple call the ioctls.
But under Linux dig around the system a little bit getting the needed
info to fill in the requested structures.
Secondly the efi_ioctl() call was adapted such that under linux it directly
read or writes out the partition table. Under Solaris this work was
handed off to the kernel via an ioctl. In the efi_write() case we also
ensure we prompt the kernel via BLKRRPART to re-scan the new partition
table. The libefi generated partition tables are correct but older
versions of ~parted-1.8.1 can not read them without a small patch.
The kernel and fdisk are able to read them just fine.
Thirdly efi_alloc_and_init() which is used by zpool to determine if a
device is a 'wholedisk' was updated to be linux aware. This check is
performed by using the partition number for the device, which the
partition number is 0 on linux it is a 'wholedisk'. However, certain
device type such as the loopback and ram disks needed to be excluded
because they do not support partitioning.
Forthly the zpool command was made symlink aware so it can correctly
resolve udev entries such as /dev/disk/by-*/*. This symlinks are
fully expanded ensuring all block devices are recognized. When a
when a 'wholedisk' block device is detected we now properly write
out an efi label and place zfs in the first partition (0th slice).
This partition is created 1MiB in to the disk to ensure it is aligned
nicely with all high end block devices I'm aware of.
This all works for me now but it did take quite a bit of work to get
it all sorted out. It would not surprise me if certain special cases
were missed so we should keep any eye of for any odd behavior.
Brian Behlendorf