MergerFS + Storage Drive disappearing on purpose (sort of "Hot Swap" action)

[HaroPo]

Hi,
I am curious about the behaviour of MergerFS in case it runs on a server / NAS without any form of RAID at all (I know, it is an unusual particular case), in use by many users, and for some reason it is necessary to disable and may be replace the drive or one of its partition for some maintenance task.

Would it be possible to tell MergerFS to ignore some branches until they are ready to re-integrate the pool ?

How should it be done ?
(I read somewhere (man page, it seems) that it is possible to do something with 'mount' regarding MergerFS binary "upgrade", would this help ?? 🤔 )

Recently, I had to use tune2fs -m 0 on Ext4 partitions to get rid of reserved blocks for 'root' user (and processes ran as 'root') on 3 different "disks" that I was sure were Not accessed by any client system while another drive still had active SSHFS mount(s) on it directly to the mount point of the drive itself, bypassing MergerFS mount point.

The 4-drive NAS setup is as follow:

# Mount points:
/mnt/ssd/0/p1/
/mnt/ssd/1/p1/
/mnt/ssd/2/p1/
/mnt/ssd/3/p1/
/mnt/mergerfs/

# MergerFS Pool
/mnt/ssd/0/p1/Backups +
/mnt/ssd/1/p1/Backups +
/mnt/ssd/2/p1/Backups +
/mnt/ssd/3/p1/Backups =
/mnt/mergerfs/Backups

Thanks for your interest and reading.

Best regards,
HP_

[trapexit]

I'm not sure what you mean "without any form of RAID". That is the normal situation. The main usage of mergerfs is you are using instead of RAID0, RAID5, etc.

I'm really not sure what you're asking. mergerfs works on paths. If the path disappears because it is unmounted then mergerfs will treat the underlying branch as it does normally. There is no special behavior. It would help if you described specific behavior.

[HaroPo]

I'm not sure what you mean "without any form of RAID". That is the normal situation. The main usage of mergerfs is you are using instead of RAID0, RAID5, etc.

I mean that there are 4 physical drives (SSD) with at least 1 partition on each ;
Each disk is mounted individually to a respective mount point / directory located in '/mnt' (see the "code" section of my initial message for illustration) ;
Each partition contains several directories with at least one having the exact same name ('Backups') that is also presented to clients / users of the NAS via MergerFS pool.

Just No RAID at all (Not Hardware, Not Software), just 4 drives in "parallel" via MergerFS merging function.

I precised this detail because "Hot Swapping" could be supported by a RAID solution hiding everything from MergerFS, as they are setups where people also involve RAID volumes on Big NAS...

I'm really not sure what you're asking. mergerfs works on paths. If the path disappears because it is unmounted then mergerfs will treat the underlying branch as it does normally.
Meaning ?

There is no special behavior. It would help if you described specific behavior.
Well, imagine MergerFS on my described setup of a NAS with 4 independent disks / partitions mounted as described.

1. What happens if one partition disappear (voluntary action in case of system maintenance) while MergerFS is active and, eventually, there is a file requested by a client system ?

1.a. Would MergerFS do its best to serve the client (in hope the requested file is on any other active and available branch, Or, "hang" ?

2. Would MergerFS refuse to work with the partition removed earlier when it is finally mounted again, Or just seamlessly allow access to the files again ?

[trapexit]

Just No RAID at all (Not Hardware, Not Software), just 4 drives in "parallel" via MergerFS merging function.

That is the normal usage of mergerfs.

What happens if one partition disappear (voluntary action in case of system maintenance) while MergerFS is active and, eventually, there is a file requested by a client system ?

Nothing happens. This is why nothing is documented about it. mergerfs doesn't care. It is path based. Not mount based. mergerfs does not care in the least. The path happens to be something you mounted or it isn't. mergerfs behaves the same. That's the point of mergerfs. To be a thin proxy. A proxy doesn't actively care.

https://trapexit.github.io/mergerfs/faq/usage_and_functionality/

[HaroPo]

Just No RAID at all (Not Hardware, Not Software), just 4 drives in "parallel" via MergerFS merging function.

That is the normal usage of mergerfs.

I think so too. :-)

What happens if one partition disappear (voluntary action in case of system maintenance) while MergerFS is active and, eventually, there is a file requested by a client system ?

Nothing happens. This is why nothing is documented about it. mergerfs doesn't care. It is path based. Not mount based. mergerfs does not care in the least. The path happens to be something you mounted or it isn't. mergerfs behaves the same. That's the point of mergerfs. To be a thin proxy. A proxy doesn't actively care.

Perfect !
Just what I expected ! :-)

https://trapexit.github.io/mergerfs/faq/usage_and_functionality/

Thanks ! 👍

[trapexit]

I'll try to make this point more explicit in the docs.