Arch Linux

pebo

Member

Registered: 2007-09-13

Posts: 15

dd/mke2fs on loopback hangs

Hi,

I posted this already on the arch mailing-list but the subject was maybe a bit daunting..

Since my friend can't reproduce this problem on Gentoo, I would like to know if someone else has the same problem. The steps are easy:

    1) create a dummy file (size: 5GB or less)
        dd if=/dev/zero of=dummy bs=1M count=5000

    2) create a loopback device on the dummy file
        losetup /dev/loop/0 dummy

    3) write on the loopback device or create a filesystem
        dd if=/dev/zero of=/dev/loop/0 bs=1M
        mke2fs -m 0 /dev/loop/0


Both commands (dd and mke2fs) should take ages to complete while hanging
around in WCHAN `congestion_wait' for most of the time.

Your reply would be perfect if you could back it with a

# dumpe2fs -h /dev/<mypartition>

of the partition on which you've created the dummy file.

Thanks in advance,

pebo

Last edited by pebo (2007-11-04 23:44:13)

retsaw

Member

From: London, UK

Registered: 2005-03-22

Posts: 132

Re: dd/mke2fs on loopback hangs

I get the "congestion wait" with the dd command on the loopback device, but not the mke2fs.

$ sudo dumpe2fs -h /dev/loop/0
dumpe2fs 1.40.2 (12-Jul-2007)
Filesystem volume name:   <none>
Last mounted on:          <not available>
Filesystem UUID:          cbcbc915-f68a-4ba4-9908-6af5dbc2afd7
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      resize_inode dir_index filetype sparse_super
Filesystem flags:         signed directory hash
Default mount options:    (none)
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              128016
Block count:              512000
Reserved block count:     0
Free blocks:              493526
Free inodes:              128005
First block:              1
Block size:               1024
Fragment size:            1024
Reserved GDT blocks:      256
Blocks per group:         8192
Fragments per group:      8192
Inodes per group:         2032
Inode blocks per group:   254
Filesystem created:       Sun Nov  4 23:53:35 2007
Last mount time:          n/a
Last write time:          Sun Nov  4 23:53:36 2007
Mount count:              0
Maximum mount count:      26
Last checked:             Sun Nov  4 23:53:35 2007
Check interval:           15552000 (6 months)
Next check after:         Sat May  3 00:53:35 2008
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Default directory hash:   tea
Directory Hash Seed:      a0f65075-e117-4a57-944f-c2623d58ae60

What do you need this output for anyway?

And why do you need to use the loopback device?  You can use mke2fs directly on a file.

Last edited by retsaw (2007-11-04 23:59:38)

pebo

Member

Registered: 2007-09-13

Posts: 15

Re: dd/mke2fs on loopback hangs

I get the "congestion wait" with the dd command on the loopback device, but not the mke2fs.

Thanks for your output. If you say, you get ``congestion_wait'' -- does that also mean that it takes long to complete? I ask because I realized that ``congestion_wait'' is not a good measure. (sometimes dd apparently hangs in c_w even if it works quite well)

And why do you need to use the loopback device?  You can use mke2fs directly on a file.

I need a block device because I would like to be able to mount the file later on.

pebo

Member

Registered: 2007-09-13

Posts: 15

Re: dd/mke2fs on loopback hangs

Eh, sorry -- I just saw that you posted sudo dumpe2fs -h /dev/loop/0, while I need dumpe2fs -h /dev/<filesystem-on-which-youve-created-the-loopback-file>..

retsaw

Member

From: London, UK

Registered: 2005-03-22

Posts: 132

Re: dd/mke2fs on loopback hangs

I get the "congestion wait" with the dd command on the loopback device, but not the mke2fs.

Thanks for your output. If you say, you get ``congestion_wait'' -- does that also mean that it takes long to complete? I ask because I realized that ``congestion_wait'' is not a good measure. (sometimes dd apparently hangs in c_w even if it works quite well)

That depends on what you consider a long time, but yes, it was a lot slower.  Typically about 7MB/s, but one run was quick at 44MB/s.

I need a block device because I would like to be able to mount the file later on.

You'll still be able to mount the filesystem later even if it not created on a block device.

Sorry, I didn't read the part about the dumpe2fs properly, here is the one from the filesystem the loopback device was on

sudo dumpe2fs -h /dev/mapper/lvm-stuff
dumpe2fs 1.40.2 (12-Jul-2007)
Filesystem volume name:   <none>
Last mounted on:          <not available>
Filesystem UUID:          af10e4bc-332a-44b0-b95d-1f6e3bfa3e80
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal resize_inode dir_index filetype needs_recovery sparse_super large_file
Filesystem flags:         signed directory hash
Default mount options:    (none)
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              53542912
Block count:              107061248
Reserved block count:     0
Free blocks:              1770338
Free inodes:              53540829
First block:              0
Block size:               4096
Fragment size:            4096
Reserved GDT blocks:      1024
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         16384
Inode blocks per group:   512
Filesystem created:       Wed Jun 20 10:56:42 2007
Last mount time:          Fri Nov  2 15:03:48 2007
Last write time:          Fri Nov  2 15:03:48 2007
Mount count:              55
Maximum mount count:      60
Last checked:             Sat Sep  1 18:13:59 2007
Check interval:           15552000 (6 months)
Next check after:         Thu Feb 28 17:13:59 2008
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               128
Journal inode:            8
Default directory hash:   tea
Directory Hash Seed:      440aa176-97fa-475a-8209-d96eff4a0c90
Journal backup:           inode blocks
Journal size:             128M

pebo

Member

Registered: 2007-09-13

Posts: 15

Re: dd/mke2fs on loopback hangs

I get the "congestion wait" with the dd command on the loopback device, but not the mke2fs.

Thanks for your output. If you say, you get ``congestion_wait'' -- does that also mean that it takes long to complete? I ask because I realized that ``congestion_wait'' is not a good measure. (sometimes dd apparently hangs in c_w even if it works quite well)

That depends on what you consider a long time, but yes, it was a lot slower.  Typically about 7MB/s, but one run was quick at 44MB/s.

The quick-one wasn't the first one, was it?

I straced the mke2fs process and saw that it always hangs after a write().. and while it does that, my firefox & vi hang too.

BTW, here is my dd-output:

# dd if=/dev/zero of=/dev/loop/0 bs=1M count=500
500+0 records in
500+0 records out
524288000 bytes (524 MB) copied, 547.585 s, 957 kB/s
                                            ^^^^^^^^

Last edited by pebo (2007-11-05 11:19:04)

retsaw

Member

From: London, UK

Registered: 2005-03-22

Posts: 132

Re: dd/mke2fs on loopback hangs

The quick-one wasn't the first one, was it?

No, it was the second.  I thought I'd test it again after the mke2fs went through quickly.  I tried it a few more times after that and the quick dd didn't repeat itself.

pebo

Member

Registered: 2007-09-13

Posts: 15

Re: dd/mke2fs on loopback hangs

Problem disappeared on 2.6.24-rc2:

http://lkml.org/lkml/2007/11/6/110

Last edited by pebo (2007-11-13 09:38:44)