On 2/29/2012 11:44 PM, Theodore Tso wrote:
You might try sorting the entries returned by readdir by inode number
before you stat them. This is a long-standing weakness in
ext3/ext4, and it has to do with how we added hashed tree indexes to
directories in (a) a backwards compatible way, that (b) was POSIX
compliant with respect to adding and removing directory entries
concurrently with reading all of the directory entries using
readdir.
When I ran into this a while back, I cobbled together this python script
to measure the correlation from name to inode, inode to first data
block, and name to first data block for all of the small files in a
large directory, and found that ext4 gives a very poor correlation due
to that directory hashing. This is one of the reasons I prefer using
dump instead of tar for backups, since it rips through my Maildir more
than 10 times faster than tar, since it reads the files in inode order.
#!/usr/bin/python
import os
from stat import *
import fcntl
import array
names = os.listdir('.')
lastino = 0
name_to_ino_in = 0
name_to_ino_out = 0
lastblock = 0
name_to_block_in = 0
name_to_block_out = 0
iblocks = list()
inode_to_block_in = 0
inode_to_block_out = 0
for file in names :
try :
st = os.stat(file)
except OSError:
continue
if not S_ISREG(st.st_mode) :
continue
if st.st_ino > lastino :
name_to_ino_in += 1
else : name_to_ino_out += 1
lastino = st.st_ino
f = open(file)
buf = array.array('I', [0])
err = fcntl.ioctl(f.fileno(), 1, buf)
if err != 0 :
print "ioctl failed on " + f
block = buf[0]
if block != 0 :
if block > lastblock :
name_to_block_in += 1
else : name_to_block_out += 1
lastblock = block
iblocks.append((st.st_ino,block))
print "Name to inode correlation: " + str(float(name_to_ino_in) / float((name_to_ino_in + name_to_ino_out)))
print "Name to block correlation: " + str(float(name_to_block_in) / float((name_to_block_in + name_to_block_out)))
iblocks.sort()
lastblock = 0
for i in iblocks:
if i[1] > lastblock:
inode_to_block_in += 1
else: inode_to_block_out += 1
lastblock = i[1]
print "Inode to block correlation: " + str(float(inode_to_block_in) / float((inode_to_block_in + inode_to_block_out)))
