数値は16進数

0xc80000000 = 50G
0x100000 = 4M
0x7d000 = 512K 

debug 2021-07-13T13:00:47.092+0000 7xxxxxxxxx00  1 bluefs _allocate unable to allocate 0x400000 on bdev 0, allocator name bluefs-wal, allocator type hybrid, capacity 0xc80000000, block size 0x100000, free 0xff000, fragmentation 0, allocated 0x0

bluestore が 50GB WAL ディスク(capacity 0xc80000000)から 4M (allocate 0x400000)の領域を確保できなかった事を意味します。
この時、512000 bytes の空きスペース(free 0x7d000)しか無かった事を意味します。

# ceph tell osd.0 perf dump | jq '.bluefs'
{
  "db_total_bytes": 10000793600000,
  "db_used_bytes": 6462504960,
  "wal_total_bytes": 26633826304,
  "wal_used_bytes": 6212812800,
  "slow_total_bytes": 0,
  "slow_used_bytes": 0,
  "num_files": 238,
  "log_bytes": 10543104,
  "log_compactions": 27922,
  "logged_bytes": 282979495936,
  "files_written_wal": 9471,
  "files_written_sst": 6428,
  "bytes_written_wal": 849981788160,
  "bytes_written_sst": 64748998656,
  "bytes_written_slow": 0,
  "max_bytes_wal": 26632781824,
  "max_bytes_db": 181695217664,
  "max_bytes_slow": 0,
  "read_random_count": 8901723,
  "read_random_bytes": 64960881964,
  "read_random_disk_count": 3069326,
  "read_random_disk_bytes": 41459603385,
  "read_random_buffer_count": 5972576,
  "read_random_buffer_bytes": 23501278579,
  "read_count": 9781511,
  "read_bytes": 193535056219,
  "read_prefetch_count": 9781457,
  "read_prefetch_bytes": 193534841109,
  "read_zeros_candidate": 0,
  "read_zeros_errors": 0
}