Re: [PATCH v2 3/3] ext4: derive f_fsid from block device to avoid collisions

[Anand Jain <anajain.sg@gmail.com>]

>> It seems we've reached the functional limits of f_fsid.
>> If we want to solve this properly for Overlayfs, NFS handles, or a
>> complex system monitoring..etc, we need a new identifier let's call
>> it f_fsid_v2, that meets the following requirements:
>>
>>   System-wide Uniqueness: Must distinguish between cloned filesystems.
>>
>>   Persistence: Must remain consistent across reboots/HW re-enumeration.

 My mistake- this should be:

  Consistency: Must remain consistent across reboots/HW re-enumeration.


>>
>>   Non-On-Disk: Must not be stored on-disk.
> 
> The third requirement doesn't make much sense to me.  If it is
> persistent it. or something it can be derived from must be stored
> on-disk.

 I hope it make sense now with "Persistence" replaced by
 "Consistency" for the 2nd requirement (above).


>> One possible implementation for f_fsid_v2 could be:
>>
>>    f_fsid_v2 =  hash(s_uuid, block_device_serial, [subvol_id])
>>
>> For pseudo block devices (virtio-blk, loop, nbd, brd,..),
>> the serial could be derived recursively:
>>
>>    serial_number = hash(backing_file.f_fsid_v2, backing_file.ino)
> 
> What i the point in this?  All of this seems to be better served
> by s_uuid.

The goal is to fix duplicate f_fsid issues in cloned filesystems
by creating a unique, reboot-consistent ID. This allows the
source and clone to remain identical (sharing same uuid) while
still being individually identifiable.

>> Note on Hardware Serials:
>>  Standard storage protocols (T10, NVMe, SAS) mandate unique,
>>  persistent serials per LUN. While I've seen T10 protocol
>>  violations during my time authoring Solaris HBA drivers, I
>>  believe these outliers shouldn't dictate the design.
> 
> No, T10 does not actually mandate unique identifiers, NVMe does, but the
> implementations are often totally broken.

Right. Newer SPC-3 (and above) compliant devices must support
the Inquiry CDB EVPD flag and provide page 0x83 for identification,
which is what we typically use for multipathing.
These are globally unique. And, we can overlook legacy
drives, as they've probably been past their EOSL for a while now.

We can tweak the algorithm as follows:

  ID = hash(s_uuid, device_identifier_id,
            partition_start_lba, partition_end_lba,
            [subvol_id], [file.ino])

This is an ID which remains consistent across reboots while
staying unique within the system, which we can use it for f_fsid.

Thanks.