zfs create -V manpage and code discrepancy

[puffi]

System information

Type	Version/Name
Distribution Name	Proxmox VE
Distribution Version	5.3-1
Linux Kernel	4.15.18-12-pve
Architecture	amd64
ZFS Version	0.7.13-pve1~bpo2
SPL Version	0.7.13-pve1~bpo2

Describe the problem you're observing

man zfs states under zfs create -V: size is automatically rounded up to the nearest 128 Kbytes to ensure that the volume has an integral number of blocks regardless of blocksize.
But the following command (with a volblocksize of 8K) zfs create -V 35K nvme/foo fails with the following error message: cannot create 'nvme/foo': volume size must be a multiple of volume block size

Describe how to reproduce the problem

Create a zvol with a size that's not aligned to the volblocksize (see command above).

[behlendorf]

Nice find. I suggest we update the code to round up as described in the documentation. We'll want to update the man page as well since the maximum blocksize can now exceed 128k.

[tapaswenipathak]

Hi @behlendorf -

I am getting confused while seeing the dir structure. I read few tickets. I know this ticket has a straight forward fix. Can you tell me how the directories are structured? or point me docs?

Few things I did -

• grep (In neither of these I find the implementation of zfs_create)

    ./cmd/zfs/zfs_main.c
    ./configure.ac
    ./include/libzfs.h
    ./include/sys/zfs_vnops.h
    ./include/sys/zfs_znode.h
    ./lib/libzfs/libzfs_dataset.c
    ./module/zfs/dsl_pool.c
    ./module/zfs/zfs_ioctl.c
    ./module/zfs/zfs_replay.c
    ./module/zfs/zfs_vnops.c
    ./module/zfs/zfs_znode.c
    ./module/zfs/zpl_inode.c
    ./module/zfs/zpl_xattr.c
  

• FAQs

  • https://github.com/zfsonlinux/zfs/wiki/FAQ

• GitHub Search - https://github.com/zfsonlinux/zfs/search?q=zfs+create&unscoped_q=zfs+create

• Wiki - https://github.com/zfsonlinux/zfs/wiki

[behlendorf]

@tapaswenipathak sure thing. The bulk of the ZFS implementation is located in module/zfs/ directory, these source files are compiled in to the zfs.ko kernel module. The main entry point on the kernel side for administrative commands, like zpool create, can be found in zfs_ioctl.c file. These operations are handled as ZFS specific ioctls issued by the user space utilities, zfs, zpool.

Specifically, the logic which check this can be found here in the zfs_ioc_create() function where zvol_check_volsize() is called. It looks like volume creation fails when it's not a multiple of the block size, which is right since we want the kernel to be strict when handling this.

You're going to want to address this by updating the zfs command line utility so it passes the volume size rounded up to the block size to the kernel. For that, you'll want to start with the zfs_do_create() function in cmd/zfs/zfs_main.c. That should get you started, thanks for looking in to this.