[Bug 898373] Re: fsck.ext3: Device or resource busy while trying to open /dev/xvda2

[Scott Moser]

I think there are 2 things that could be causing this from cloud-init:
 a.) cloudinit/CloudConfig/cc_resize.py [1]
   This uses blkid, but almost certainly only passes blkid a device node that represents '/'.  It basically does a stat on /, gets major and minor, creates a device node that has the same major, minor and calls blkid on that like:
    blkid -c /dev/null -sTYPE -ovalue /tmp/THATDEV
   It does this to find the filesystem type of /.

 b.) cloudinit/DataSourceOVF.py [2]
   This would run early in the boot, and possibly race with fsck.  It is trying to determine if each device in /dev matching '^(sr[0-9]+|hd[a-z]|xvd.*)' is a cdrom.  Notable in that list is 'xvd.*'.  If that a device matching that is not mounted, then we do a read(512) in an attempt to determine if there is media in the CD-ROM drive, and if *that* succeeds, it will try to mount read-only and look for some files, then unmount.

It seems to me that 'b' is a potential source of the issue, as either
the 'read()' or the mount, check, umount could cause the resource busy
option.

2 things that could address that:
 a.) restrict devices being checked to 'xvd[a-z]' (eliminating the possible mount of /dev/xvda[0-9] entirely.  This could still cause issue on other devices like /dev/xvdb.
 b.) mount with 'ro,noload' (per 'man mount')

I'm *guessing* that fsck is trying to open the device in read-only and
the mount in 'b' is not truely read-only.

Any thoughts on how I can essentially poll these devices looking for "ovf data" and avoid this race?
Woudl a badly timed 'read()' interfere with fsck ? (ie, if it was an open RO, not RW?)

One thing that would filter out this device would be to filter out non-
CD ROM devices, but I actually like the fact that the device does not
have to be a CDROM.


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[1] http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/precise/cloud-init/precise/view/head:/cloudinit/CloudConfig/cc_resizefs.py
[2] http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/precise/cloud-init/precise/view/head:/cloudinit/DataSourceOVF.py

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https://bugs.launchpad.net/bugs/898373

Title:
  fsck.ext3: Device or resource busy while trying to open /dev/xvda2

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