Have you read through available documentation, open Github issues and Github Q&A Discussions?

Yes

System information

Debian 11.7 (amd64) + Proxmox 7.4 / MooseFS Pro 3.0.117

root@srv:~# uname -a
Linux srv 5.15.116-1-pve #1 SMP PVE 5.15.116-1 (2023-08-29T13:46Z) x86_64 GNU/Linux

Hardware / network configuration, and underlying filesystems on master, chunkservers, and clients.

Underlying fs: ext4

2 x master server
2 x chunkservers
2 x clients

How much data is tracked by moosefs master (order of magnitude)?

Empty, new MooseFS.

Describe the problem you observed.

I try to use mfsbdev for virtual machine images. But before i start use this for vm, i have a problem:
When i start mfsbdev and 1. map file 2. unmap file 3. map file again , then ndb device is not working properly anymore.

If i stop and start mfsbdev, map is start normally and working ok. But if i have several vm`s, then i can not stop mfsbdev service in production ready system, because i need to stop all virtual machines, in this case.

1. VM is stopped (vm is not important now).

2. map + unmap + map uses only /dev/nbd0

3. Additional settings:

root@srv:~# cat /etc/modprobe.d/nbd.conf
options nbd nbds_max=64

Can you reproduce it? If so, describe how. If not, describe troubleshooting steps you took before opening the issue.

Yes:

1. map + unmap + map

root@srv:~# modprobe nbd

root@srv:~# /usr/sbin/mfsbdev start -H mfsmaster.lan
mfsmaster accepted connection with parameters: read-write,restricted_ip,admin ; root mapped to root:root

root@srv:~# /usr/sbin/mfsbdev map -f vz/nbd/100/vm-100.bin -s 32GB -n vm-wire-lv
started block device: (/dev/mfs/vm-wire-lv->/dev/nbd0 : MFS:/vz/nbd/100/vm-100.bin : 29.802GiB)

root@srv:~# fdisk /dev/mfs/vm-wire-lv

Welcome to fdisk (util-linux 2.36.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): p
Disk /dev/mfs/vm-wire-lv: 29.8 GiB, 32000000000 bytes, 7812500 sectors
Units: sectors of 1 * 4096 = 4096 bytes
Sector size (logical/physical): 4096 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x33c0c8a3

Device               Boot Start      End  Sectors   Size Id Type
/dev/mfs/vm-wire-lv1 *     2048 62498815 62496768 238.4G 83 Linux

Command (m for help): q

root@srv:~# mfsbdev unmap -f vz/nbd/100/vm-100.bin
stop block device: (/dev/mfs/vm-wire-lv->/dev/nbd0 : MFS:/vz/nbd/100/vm-100.bin : 29.802GiB)

root@srv:~# /usr/sbin/mfsbdev map -f vz/nbd/100/vm-100.bin -s 32GB -n vm-wire-lv
started block device: (/dev/mfs/vm-wire-lv->/dev/nbd0 : MFS:/vz/nbd/100/vm-100.bin : 29.802GiB)

root@srv:~# fdisk /dev/mfs/vm-wire-lv

Welcome to fdisk (util-linux 2.36.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

fdisk: cannot open /dev/mfs/vm-wire-lv: Input/output error
root@srv:~# cfdisk /dev/mfs/vm-wire-lv
cfdisk: cannot open /dev/mfs/vm-wire-lv: Input/output error

Also strange size of partition = 238.4G in 32GB disk
/dev/mfs/vm-wire-lv1 * 2048 62498815 62496768 238.4G 83 Linux

2. map + unmap + map + unmap + map (Without restarting mfsbdev service)
   After map + unmap + map and get input/output error, then if try to repeat additional unmap + map, then working ok, without restart mfsbdev service.

root@srv:~# mfsbdev unmap -f vz/nbd/100/vm-100.bin
stop block device: (/dev/mfs/vm-wire-lv->/dev/nbd1 : MFS:/vz/nbd/100/vm-100.bin : 29.802GiB)

root@srv:~# /usr/sbin/mfsbdev map -f vz/nbd/100/vm-100.bin -s 32GB -n vm-wire-lv
started block device: (/dev/mfs/vm-wire-lv->/dev/nbd1 : MFS:/vz/nbd/100/vm-100.bin : 29.802GiB)

root@srv:~# fdisk /dev/mfs/vm-wire-lv

Welcome to fdisk (util-linux 2.36.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help):

And if try to repeat one more unmap + map, then get again input/output error.

I mean:

map = ok
map + unmap + map = input/output error
map + unmap + map + unmap + map = ok
map + unmap + map + unmap + map + unmap + map = input/output error
map + unmap + map + unmap + map + unmap + map + unmap + map = ok

Additionally:

Also i try to use -i for ignore locking, but this is not working too. I have an idea - map single image on 2 different servers with -i key for ready hot standby scenario for vm`s, but if i stop vm on 1-st server and unmap file of vm on 1-st server, then on 2-nd server i also see input/output error when i try to use fdisk/cfdisk.

I have two servers in MooseFS Pro cluster with mfsbdev and want to have hot standby scenario for virtual machines with -i key (ideal solution with pre-mapped image files on 2-nd server) or without -i key, if this is impossible to have more than 1 ndb mapped devices with single image on 2 different servers in cluster - in this case i can manually map ndb devices before start vm`s on 2-nd server if 1-st server is going down.

But now, simple map + unmap + map = broken ndb device on single server too.

After stopping mfsbdev and removing nbd module from kernel, i also see stuck connections:

root@srv:~# ps aux | grep "nbd\|bdev" | grep -v grep ; lsmod | grep nbd
root         883  0.0  0.0      0     0 ?        I<   Sep10   1:36 [kworker/u196:0-nbd0-recv]
root     1474426  0.0  0.0      0     0 ?        I<   07:25   0:00 [kworker/u196:1-nbd1-recv]