When there are several requests inflight a server may grant a client with more grants than it requests.

In real environment it has been reported that clients were granted ~800mb while max_dirty_mb set to 256.

$ cat compute_clients_cur_grant_bytes.txt  | grep OST019c | awk -F= 'BEGIN \{ grants=0 } \{ if (grants < $2) grants = $2 } END \{ print grants }'
805306368

In total clients were granted ~960gb

$ cat compute_clients_cur_grant_bytes.txt  | grep OST019c | awk -F= '{ grants += $2 } END { print grants }'
971171270656

That resulted in ENOSPC condition when osts had about 1tb of free space.