I have managed to find a way to do this. It uses a python script which first uses
debugfs to find the necesssary number of blocks (including indirect blocks) that the file will need. It then manually writes the indirect blocks to the disk, and invokes
debugfs again to mark the blocks as used and to update the file's inode.
The only issue is that
debugfs apparently doesn't update the free block count of the block group when you use
setb. Although I can set that parameter manually, there doesn't appear to be any way to print the current value so I can't calculate the correct value. As far as I can tell it doesn't have any real negative consequences, and
fsck.ext3 can be used to correct the values if needed, so for benchmark purposes it'll do.
If there's any other file system consistency issue I've missed, please let me know, but since
fsck.ext3 reports nothing besides the incorrect free block count I should be safe.
SECTOR_SIZE = 512
BLOCK_SIZE = 4096
DIRECT_BLOCKS = 12
BLOCKS_PER_INDIRECT_BLOCK = BLOCK_SIZE / 4
def write_indirect_block(device, indirect_block, blocks):
print "writing indirect block ", indirect_block
dev = open(device, "wb")
dev.seek(indirect_block * BLOCK_SIZE)
# Write blocks
for block in blocks:
bin_block = struct.pack("<I", int(block))
zero = struct.pack("<I", 0)
# Zero out the rest of the block
for x in range(len(blocks), BLOCKS_PER_INDIRECT_BLOCK):
if len(argv) < 5:
print "Usage: ext3allocfile.py [device] [file] [sizeInMB] [offsetInMB]"
device = argv # device containing the ext3 file system, e.g. "/dev/sdb1"
file = argv # file name relative to the root of the device, e.g. "/myfile"
size = int(argv) * 1024 * 1024 # Size in MB
offset = int(argv) * 1024 * 1024 # Offset from the start of the device in MB
if size > 0xFFFFFFFF:
# Supporting this requires two things: triple indirect block support, and proper handling of size_high when changing the inode
print "Unable to allocate files over 4GB."
# Because size is specified in MB, it should always be exactly divisable by BLOCK_SIZE.
size_blocks = size / BLOCK_SIZE
# We need 1 indirect block for each 1024 blocks over 12 blocks.
ind_blocks = (size_blocks - DIRECT_BLOCKS) / BLOCKS_PER_INDIRECT_BLOCK
if (size_blocks - DIRECT_BLOCKS) % BLOCKS_PER_INDIRECT_BLOCK != 0:
ind_blocks += 1
# We need a double indirect block if we have more than one indirect block
has_dind_block = ind_blocks > 1
total_blocks = size_blocks + ind_blocks
total_blocks += 1
# Find free blocks we can use at the offset
offset_block = offset / BLOCK_SIZE
print "Finding ", total_blocks, " free blocks from block ", offset_block
process = subprocess.Popen(["debugfs", device, "-R", "ffb %d %d" % (total_blocks, offset_block)], stdout=subprocess.PIPE)
output = process.stdout
# The first three entries after splitting are "Free", "blocks", "found:", so we skip those.
blocks = output.readline().split(" ")[3:]
# The last entry may contain a line-break. Removing it this way to be safe.
blocks = filter(lambda x: len(x.strip(" \n")) > 0, blocks)
if len(blocks) != total_blocks:
print "Not enough free blocks found for the file."
# The direct blocks in the inode are blocks 0-11
# Write the first indirect block, listing the blocks for file blocks 12-1035 (inclusive)
if ind_blocks > 0:
write_indirect_block(device, int(blocks[DIRECT_BLOCKS]), blocks[DIRECT_BLOCKS + 1 : DIRECT_BLOCKS + 1 + BLOCKS_PER_INDIRECT_BLOCK])
dind_block_index = DIRECT_BLOCKS + 1 + BLOCKS_PER_INDIRECT_BLOCK
dind_block = blocks[dind_block_index]
ind_block_indices = [dind_block_index+1+(i*(BLOCKS_PER_INDIRECT_BLOCK+1)) for i in range(ind_blocks-1)]
# Write the double indirect block, listing the blocks for the remaining indirect block
write_indirect_block(device, int(dind_block), [blocks[i] for i in ind_block_indices])
# Write the remaining indirect blocks, listing the relevant file blocks
for i in ind_block_indices:
write_indirect_block(device, int(blocks[i]), blocks[i+1:i+1+BLOCKS_PER_INDIRECT_BLOCK])
# Time to generate a script for debugfs
script = tempfile.NamedTemporaryFile(mode = "w", delete = False)
# Mark all the blocks as in-use
for block in blocks:
script.write("setb %s\n" % (block,))
# Change direct blocks in the inode
for i in range(DIRECT_BLOCKS):
script.write("sif %s block[%d] %s\n" % (file, i, blocks[i]))
# Change indirect block in the inode
if size_blocks > DIRECT_BLOCKS:
script.write("sif %s block[IND] %s\n" % (file, blocks[DIRECT_BLOCKS]))
# Change double indirect block in the inode
script.write("sif %s block[DIND] %s\n" % (file, dind_block))
# Set total number of blocks in the inode (this value seems to actually be sectors
script.write("sif %s blocks %d\n" % (file, total_blocks * (BLOCK_SIZE / SECTOR_SIZE)))
# Set file size in the inode
# TODO: Need support of size_high for large files
script.write("sif %s size %d\n" % (file, size))
# execute the script
print "Modifying file"
subprocess.call(["debugfs", "-w", device, "-f", script.name])
if __name__ == "__main__":
The script can be used as follows to create a 1GB file at offset 200GB (you need to be root):
python ext3allocfile.py /dev/sdb1 /myfile 1024 204800
The umount/mount combo is necessary to get the system to recognize the change. You can unmount before invoking the script but that makes invoking
If anyone wants to use this: I don't guarantee it'll work right, I don't take responsibility if you lose any data. In general, don't use it on a file system that contains anything important.