Dear Ceph users,
I'd need some help in understanding the total space in a CephFS. My
cluster is currently built of 8 machines, the one with the smallest
capacity has 8 TB of total disk space, and the total available raw space
is 153 TB. I set up a 3x replicated metadata pool and a 6+2 erasure
coded data pool with host failure domain for my CephFS. In this
configuration every host holds a data chunk, so I would expect a total
of about 48 TB of total storage space. I computed this value by noting
that (roughly speaking and neglecting the metadata) 48 TB of data will
need 48 TB of data chunks and 16 TB of coding chunks, for a total of 64
TB that evenly divided into my 8 machines gives an occupancy of 8 TB per
host, which exactly saturates the smallest one.
Assuming that the above is correct then I would expect that a df -h on a
machine mounting the CephFS would report 48 TB of total space. Instead
it started with something around 75 TB at the beginning, and it's slowly
decreasing while I'm transferring data to the CephFS, being now at 62 TB.
I cannot understand this behavior, nor if my assumptions about the total
space are correct, so I'd need some help with this.
Thanks,
Nicola
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