Damn i guess this does not work (due to insufficent space?):

May  8 19:07:34 xen-01-b SMGC: [8597] Set vhd-blocks = eJzt2LEJgEAMhtHS0tFvFMdwHC0FA9ENDkEJeO/Vge+vs+TtzGyZkT0xdU8+tc+1/SjuH8X9tTYPAAAAAAAAAAAAAMAzrXoAADCcbfA+AAAA8H/+D++5AMbVs9Q= for *ef6aa466[VHD](300.000G//300.504G|n)
May  8 19:07:34 xen-01-b SMGC: [8597] Num combined blocks = 153555
May  8 19:07:34 xen-01-b SMGC: [8597] Coalesced size = 300.504G
May  8 19:07:34 xen-01-b SMGC: [8597] Got other-config for b328d061[VHD](300.000G/242.496G/300.594G|n): {'leaf-coalesce': 'offline', 'content_id': 'c4cec5e9-e6f4-18a9-ad52-8db03ef0f2c9'}
May  8 19:07:34 xen-01-b SMGC: [8597] No space to leaf-coalesce b328d061[VHD](300.000G/242.496G/300.594G|n) (free space: 254355177472)
May  8 19:07:34 xen-01-b SMGC: [8597] ...but enough space if skip snap-coalesce
May  8 19:07:34 xen-01-b SMGC: [8597] Set leaf-coalesce = offline for b328d061[VHD](300.000G/242.496G/300.594G|n)
May  8 19:07:34 xen-01-b SMGC: [8597] Got sm-config for *ef6aa466[VHD](300.000G//300.504G|n): {'read-caching-reason-24097d1c-c678-417e-8a00-2257aee78fd2': 'SR_NOT_SUPPORTED', 'read-caching-enabled-on-24097d1c-c678-417e-8a00-2257aee78fd2': 'false', 'vdi_type': 'vhd', 'vhd-blocks': 'eJzt2LEJgEAMhtHS0tFvFMdwHC0FA9ENDkEJeO/Vge+vs+TtzGyZkT0xdU8+tc+1/SjuH8X9tTYPAAAAAAAAAAAAAMAzrXoAADCcbfA+AAAA8H/+D++5AMbVs9Q='}
May  8 19:07:34 xen-01-b SMGC: [8597] No work, exiting
May  8 19:07:34 xen-01-b SMGC: [8597] GC process exiting, no work left

I dont fully understand that, nothing changed?!:

May  8 19:07:34 xen-01-b SMGC: [8597] SR 646a ('Local SR R10 - 900GB') (2 VDIs in 1 VHD trees): showing only VHD trees that changed:
May  8 19:07:34 xen-01-b SMGC: [8597]         *ef6aa466[VHD](300.000G//300.504G|n)
May  8 19:07:34 xen-01-b SMGC: [8597]             b328d061[VHD](300.000G/242.496G/300.594G|n)
May  8 19:07:34 xen-01-b SMGC: [8597]

Oh! great, i will try that later when the server is less busy.
Can i trigger it (coalesce) with an SR rescan ?

Well without XOA Backup working i have no other option as to shutdown and export that VM?
I just noticed that XCP still reports "Cannot move virtual disks between local SRs" (havnt tried that for a long time).

This is the storage in question

I am facing a simmilar issue like this one https://xen-orchestra.com/forum/topic/1192/base-copy/
I have a 900GB local SR with a single 300GB VM on it, and now my Backup fails with SR_BACKEND_FAILURE_44

So there is a 300GB VDI and the 300GB Base Copy on that storage, so from what i understand the remaining ~260GB are NOT engough space to merge these 2 copys ?!

xe vdi-list sr-uuid=646a7969-1ec5-66b0-e7d2-cc76a340b171
uuid ( RO)                : b328d061-f9c0-4080-91bb-872c36fe6e76
          name-label ( RW): bitrix.np.disk1
    name-description ( RW): Disk Image
             sr-uuid ( RO): 646a7969-1ec5-66b0-e7d2-cc76a340b171
        virtual-size ( RO): 322122547200
            sharable ( RO): false
           read-only ( RO): false


uuid ( RO)                : ef6aa466-72ba-4430-b3d1-0fb7ddae6079
          name-label ( RW): base copy
    name-description ( RW):
             sr-uuid ( RO): 646a7969-1ec5-66b0-e7d2-cc76a340b171
        virtual-size ( RO): 322122547200
            sharable ( RO): false
           read-only ( RO): true

I have tried to use "reclaim space" on that SR, but that fails for me (and alot of others aparently) with:

FAILED in util.pread: (rc 1) stdout: '', stderr: 'blkdiscard: /dev/VG_XenStorage-646a7969-1ec5-66b0-e7d2-cc76a340b171/646a7969-1ec5-66b0-e7d2-cc76a340b171_trim_lv: BLKDISCARD ioctl failed: Operation not supported

I see alot of exceptions in SMlog:

May  7 09:34:08 xen-01-b SM: [7866] Ignoring exception for LV check: /dev/VG_XenStorage-bb0f6287-2108-3772-928e-8048c1e8d868/bb0f6287-2108-3772-928e-8048c1e8d868_trim_lv !
May  7 09:36:23 xen-01-b SM: [8838] Ignoring exception for LV check: /dev/VG_XenStorage-646a7969-1ec5-66b0-e7d2-cc76a340b171/646a7969-1ec5-66b0-e7d2-cc76a340b171_trim_lv !
May  7 09:45:11 xen-01-b SM: [12626] Ignoring exception for LV check: /dev/VG_XenStorage-95ec05c5-59bd-888e-eddb-48d635b96116/95ec05c5-59bd-888e-eddb-48d635b96116_trim_lv !
May  7 10:05:05 xen-01-b SM: [20938] Ignoring exception for LV check: /dev/VG_XenStorage-646a7969-1ec5-66b0-e7d2-cc76a340b171/b328d061-f9c0-4080-91bb-872c36fe6e76.cbtlog !
May  7 10:05:17 xen-01-b SM: [21436] Ignoring exception for LV check: /dev/VG_XenStorage-646a7969-1ec5-66b0-e7d2-cc76a340b171/b328d061-f9c0-4080-91bb-872c36fe6e76.cbtlog !

So what is the fix here ? Do i need to resize that VDI to fit 3 Times on the local storage ?