CEPH: how to create an OSD (bluestore) on one disk and place block.db on a partition of another disk?

Question

linux

/dev/sdc - HDD
/dev/sdm - SSD with multiple partitions, the first is the OS (Ubuntu Server 16.04.3 LTS, ceph 12.2.0 luminous), the rest (5,6,7,8) are empty partitions.
I do this:
ceph-disk prepare --bluestore /dev/sdc --block.db /dev/sdm6
Here is the output:

setting name!
partNum is 0
REALLY setting name!
The operation has completed successfully.
prepare_device: OSD will not be hot-swappable if block.db is not the same device as the osd data
prepare_device: Block.db /dev/sdm6 was not prepared with ceph-disk. Symlinking directly.
setting name!
partNum is 1
REALLY setting name!
The operation has completed successfully.
The operation has completed successfully.
meta-data=/dev/sdc1 isize=2048 agcount=4, agsize=6400 blks
= sectsz=4096 attr=2, projid32bit=1
= crc=1 finobt=1, sparse=0
data = bsize=4096 blocks=25600, imaxpct=25
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal log bsize=4096 blocks=1608, version=2
= sectsz=4096 sunit=1 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
Warning: The kernel is still using the old partition table.
The new table will be used at the next reboot or after you
run partprobe(8) or kpartx(8)
The operation has completed successfully.

There are no errors.
Now doing
ceph-disk activate-all
here is the output:

got monmap epoch 1
2017-09-12 14:07:33.711863 7f0fba54be00 -1 bdev(0x55b99f8c6fc0 /var/lib/ceph/tmp/mnt.vyVmpd/block.db) open open got: (13) Permission denied
2017-09- 12 14:07:33.711875 7f0fba54be00 -1 bluestore(/var/lib/ceph/tmp/mnt.vyVmpd) _open_db add block device(/var/lib/ceph/tmp/mnt.vyVmpd/block.db) returned: (13 ) Permission denied 2017-09-12
01:07:33.993025 7f0fba54be00 -1
-1 OSD::mkfs: ObjectStore::mkfs failed with error (13) Permission denied
2017-09-12 14:07:33.993275 7f0fba54be00 /mnt.vyVmpd: (13) Permission denied
mount_activate: Failed to activate
ceph-disk: Command '['/usr/bin/ceph-osd', '--cluster', 'ceph', '--mkfs', '-i', u'2', '--monmap', '/var/lib/ceph/tmp/mnt.vyVmpd/activate.monmap', '--osd-data', '/var/lib/ceph/tmp/mnt.vyVmpd', '--osd-uuid', u'1b0a0046-1cca-4684-93a2-8012320109d8', '--setuser', 'ceph', '--setgroup', 'ceph']' returned non-zero exit status 1
Removed symlink /etc/systemd/system/ ceph-osd.target.wants/[email protected]
Created symlink from /etc/systemd/system/ceph-osd.target.wants/[email protected] to /lib/systemd/system/[email protected]
Removed symlink /etc/systemd/system/ceph-osd.target.wants/[email protected]
Created symlink from /etc/systemd/system/ceph-osd.target.
ceph-disk: Error: One or more partitions failed to activate

What am I doing wrong, comrades?