Question about migrating from iSCSI to RBD

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Hello!

I was hoping to inquire if anyone here has attempted similar operations, and if they ran into any issues. To give a brief overview of my situation, I have a standard octopus cluster running 15.2.2, with ceph-iscsi installed via ansible. The original scope of a project we were working on changed, and we no longer need the iSCSI overhead added to the project (the machine using CEPH is Linux, so we would like to use native RBD block devices instead).

Ideally we would create some new pools and migrate the data from the iSCSI pools over to the new pools, however, due to the massive amount of data (close to 200 TB), we lack the physical resources necessary to copy the files.

Digging a bit on the backend of the pools utilized by ceph-iscsi, it appears that the iSCSI utility uses standard RBD images on the actual backend:

~]# rbd info iscsi/pool-name
rbd image 'pool-name':
    size 200 TiB in 52428800 objects
    order 22 (4 MiB objects)
    snapshot_count: 0
    id: 137b45a37ad84a
    block_name_prefix: rbd_data.137b45a37ad84a
    format: 2
    features: layering, exclusive-lock, object-map, fast-diff, deep-flatten
    op_features:
    flags: object map invalid, fast diff invalid
    create_timestamp: Thu Nov 12 16:14:31 2020
    access_timestamp: Tue Mar 16 16:13:41 2021
    modify_timestamp: Tue Mar 16 16:15:36 2021

And I can also see that, like a standard rbd image, our 1st iSCSI gateway currently holds the lock on the image:

]# rbd lock ls --pool iscsi pool-name
There is 1 exclusive lock on this image.
Locker          ID              Address
client.3618592  auto 259361792  10.101.12.61:0/1613659642

Theoretically speaking, would I be able to simply stop & disable the tcmu-runner processes on all iSCSI gateways in our cluster, which would release the lock on the RBD image, then create another user with rwx permissions to the iscsi pool? Would this work, or am I missing something that would come back to bite me later on?

Looking for any advice on this topic. Thanks in advance for reading!

--

Justin Goetz
Systems Engineer, TeraSwitch Inc.
jgoetz@xxxxxxxxxxxxxx
412-945-7045 (NOC) | 412-459-7945 (Direct)

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