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· 2 min read
Joachim Kraftmayer

ceph-volume can be used to create for a existing OSD a new WAL/DB on a faster device without the need to recreate the OSD.

ceph-volume lvm new-db --osd-id 15 --osd-fsid FSID --target cephdb/cephdb1
--> NameError: name 'get_first_lv' is not defined
this is a bug in ceph-volume v16.2.7 that will be fixed in v16.2.8 
[https://github.com/ceph/ceph/pull/44209](https://github.com/ceph/ceph/pull/44209)

First create a new Logical Volume on the Device that will hold the new WAL/DB

vgcreate cephdb /dev/sdb
Volume group "cephdb" successfully created
lvcreate -L 100G -n cephdb1 cephdb
Logical volume "cephdb1" created.

Now stop running OSD and if it was deactivated ( cephadm ) then activate it on the host

systemctl stop ceph-FSID@osd.0.service
ceph-volume lvm activate --all --no-systemd

Create new WAL/DB on new Device

ceph-volume lvm new-db --osd-id 0 --osd-fsid OSD-FSID --target cephdb/cephdb1
--> Making new volume at /dev/cephdb/cephdb1 for OSD: 0 (/var/lib/ceph/osd/ceph-0)
Running command: /bin/chown -h ceph:ceph /var/lib/ceph/osd/ceph-0/block.db
Running command: /bin/chown -R ceph:ceph /dev/dm-1
--> New volume attached.

Migrate existing WAL/DB to new Device

ceph-volume lvm migrate --osd-id 0 --osd-fsid OSD-FSID --from data --target cephdb/cephdb1
--> Migrate to existing, Source: ['--devs-source', '/var/lib/ceph/osd/ceph-0/block'] Target: /var/lib/ceph/osd/ceph-0/block.db
--> Migration successful.

Deactivate OSD and start it

ceph-volume lvm deactivate 0
Running command: /bin/umount -v /var/lib/ceph/osd/ceph-0
 stderr: umount: /var/lib/ceph/osd/ceph-0 unmounted
systemctl start ceph-FSID@osd.0.service

· One min read
Joachim Kraftmayer

When configuring osd in mixed setup with db and wal colocated on a flash device, ssd or NVMe. There were always changes and irritations where the DB and the WAL are really located. With a simple test it can be checked: The location of the DB for the respective OSD can be verified via ceph osd metadata osd.<id> and the variable "bluefs_dedicated_db": "1".

The WAL was created separately in earlier Ceph versions and automatically on the same device as the DB in later Ceph versions. The WAL can be easily tested by using the ceph osd.<id> tell bench command.

First you check larger write operations with the command:

ceph tell osd.0 bench 65536 409600

Second, you check with smaller objects that are smaller than the bluestore_prefer_deferred_size_hdd (64k).

ceph tell osd.0 bench 65536 4096

If you compare the IOPs of the two tests, one result should correspond to the IOPs of an SSD and the other result should be quite low for the HDD. From this you can know if the WAL is on the HDD or the flash device.