python3 -c "
import struct

# Read superblock
with open('/dev/md0','rb') as f:
    f.seek(2621441024)  # physical primary sb
    sb = f.read(1024)

total_blocks = struct.unpack_from('<I', sb, 4)[0]
bpg = struct.unpack_from('<I', sb, 40)[0]
bsize = 4096
num_groups = (total_blocks + bpg - 1) // bpg
gdt_blocks = (num_groups * 64 + bsize - 1) // bsize  # 64 bytes per GDT entry

print(f'Total blocks:  {total_blocks}')
print(f'Blocks/group:  {bpg}')
print(f'Num groups:    {num_groups}')
print(f'GDT size:      {num_groups * 64} bytes = {gdt_blocks} blocks')
print(f'GDT spans blocks 1 to {gdt_blocks}')
print()

# Check which physical chunks cover the GDT
CHUNK_BYTES = 128 * 512  # 64KB
LV_PHYS_START = 5120000 * 512

def v_to_p_byte(virt_byte):
    group = virt_byte // (5 * CHUNK_BYTES)
    offset_in_group = virt_byte % (5 * CHUNK_BYTES)
    chunk_in_group = offset_in_group // CHUNK_BYTES
    intra = offset_in_group % CHUNK_BYTES
    if chunk_in_group == 4:
        return None  # metadata chunk
    phys = (LV_PHYS_START +
            group * 4 * CHUNK_BYTES +
            chunk_in_group * CHUNK_BYTES +
            intra)
    return phys

# GDT spans virtual bytes 4096 to 4096+gdt_blocks*4096
gdt_start_v = 4096   # block 1
gdt_end_v = 4096 + gdt_blocks * 4096

print(f'GDT virtual bytes: {gdt_start_v} to {gdt_end_v}')
print()

# Check each chunk that covers the GDT
print('Chunks covering GDT:')
pos = gdt_start_v
while pos < gdt_end_v:
    chunk_in_group = (pos % (5 * CHUNK_BYTES)) // CHUNK_BYTES
    phys = v_to_p_byte(pos)
    chunk_end = pos + CHUNK_BYTES - (pos % CHUNK_BYTES)
    print(f'  Virtual {pos}-{min(chunk_end,gdt_end_v)}: '
          f'chunk_type={chunk_in_group} '
          f'physical={phys} '
          f'{\"METADATA-LOST\" if phys is None else \"\"}')
    pos = chunk_end

# Read GDT from nbd and check first few entries
print()
print('GDT entries via NBD:')
with open('/dev/nbd0','rb') as f:
    f.seek(4096)
    gdt_data = f.read(min(gdt_blocks * 4096, 65536))

for i in range(min(10, num_groups)):
    entry = gdt_data[i*64:(i+1)*64]
    bb = struct.unpack_from('<I', entry, 0)[0]
    ib = struct.unpack_from('<I', entry, 4)[0]
    it = struct.unpack_from('<I', entry, 8)[0]
    cs = struct.unpack_from('<H', entry, 30)[0]
    print(f'  Group {i}: bb={bb} ib={ib} it={it} csum=0x{cs:04x}')
"