I'm trying to get my Linux-based NTFS backup drive to pass a CHKDSK and came
upon this curious situation where CHKDSK finds errors.
It seems to be some issue with how ntfs-3g modifies a directory index when
renaming many files.
The CHKDSK error always seems to be of the form:
Stage 2: Examining file name linkage ...
The first free byte, 0xc0, and bytes available, 0x150, for root index $I30 in
file 0x40 are not equal.
I've attached a python script (mkbaddir.py) that creates two (apparently)
identical directories, one of which reliably causes this CHKDSK error; the
other doesn't.
How to demonstrate:
- Format an NTFS partition or thumbdrive using Windows or mkfs.ntfs.
- Mount the partition on a Linux system.
I used Mint 20 with ntfs-3g 2017.3.23AR.3 integrated FUSE 28 and
python 3.8.2.
- Chdir to the new NTFS partition and run the script:
/tmp/mkbaddir.py # creates 'baddir' in current
dir.
/tmp/mkbaddir.py -G # creates 'gooddir' in current dir.
diff -r baddir gooddir # no difference
du -sB1 baddir gooddir # same size (128K)
- Boot into Windows (10 v1903) and run (from a terminal) chkdsk X:
(where X: is the NTFS drive).
- This will say:
"Errors found. CHKDSK cannot continue in read-only mode."
- Delete baddir (I used cygwin's rm -rf), and run chkdsk X: again.
- This will now have no errors.
My guess at what's happening:
The script creates a directory of 410 empty files and then renames them with
slightly larger names, which as I understand leaves a bunch of unused nodes in
the b-tree. The -G option just renames the 410 known files; without the -G
option, it uses os.walk() to traverse the directory which I'm guessing leaves
the b-tree in a slightly different state with even more unused nodes.
The 410 was chosen by trial-and-error so that some internal threshhold is just
exceeded by the baddir but not by the gooddir. With more than 410 (using the
-c option; say -c 500), both baddir and gooddir will cause CHKDSK errors.
If I run the script on Windows/cygwin (Python 3.6.9) to create the folders, it
does not give any CHKDSK errors even with many more files.
So there seems to be some issue with how ntfs-3g modifies the b-tree when
renaming many files that is causing CHKDSK to complain.
I encountered this issue when trying to get my Linux-based NTFS backup drive to
consistently pass a CHKDSK. I use a script to first rename POSIX names to
valid windows names, replacing '?' with '@@3F', etc so I can reverse the
renaming afterwards. I have some website mirror folders with many files of the
form:
details.asp?id=xxxxx&key=val
which gave rise to this issue. (In the mkbaddir script I use only
alphanumeric names to be clear this is not an illegal char issue).
--------------------- mkbaddir.py
------------------------------------------------
#!/usr/bin/python3
import os, re, argparse
def mkname(i): return "detaildetaildetail-N-%04d" % i
parser = argparse.ArgumentParser()
parser.add_argument('-G', '--good', action='store_true')
parser.add_argument( '-c', '--count', action='store', default=410)
opts = parser.parse_args()
count = int(opts.count)
if opts.good: dirname = 'gooddir'
else: dirname = 'baddir'
# Create a dir of files
os.mkdir(dirname)
for i in range(count):
f = dirname + '/' + mkname(i)
open(f,'a').close() # touch
# rename them
if opts.good:
for i in range(count):
f = mkname(i)
nf = re.sub('N', "KK3F", f)
os.rename(dirname+'/'+f, dirname+'/'+nf)
else:
for dirpath, dirs, files in os.walk(dirname, topdown=False):
for f in files:
nf = re.sub('N', "KK3F", f)
os.rename(dirname+'/'+f, dirname+'/'+nf)
#!/usr/bin/python3
import os, re, argparse
def mkname(i): return "detaildetaildetail-N-%04d" % i
parser = argparse.ArgumentParser()
parser.add_argument('-G', '--good', action='store_true')
parser.add_argument( '-c', '--count', action='store', default=410)
opts = parser.parse_args()
count = int(opts.count)
if opts.good: dirname = 'gooddir'
else: dirname = 'baddir'
# Create a dir of files
os.mkdir(dirname)
for i in range(count):
f = dirname + '/' + mkname(i)
open(f,'a').close() # touch
# rename them
if opts.good:
for i in range(count):
f = mkname(i)
nf = re.sub('N', "KK3F", f)
os.rename(dirname+'/'+f, dirname+'/'+nf)
else:
for dirpath, dirs, files in os.walk(dirname, topdown=False):
for f in files:
nf = re.sub('N', "KK3F", f)
os.rename(dirname+'/'+f, dirname+'/'+nf)
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