July 2012 - Libguestfs - Libguestfs List Archives

[PATCH 0/4] Provide guestmount --pid-file and document possible race when unmounting FUSE filesystems.

by Richard W.M. Jones

The full description of this bug is here: https://bugzilla.redhat.com/show_bug.cgi?id=838592 and the effect it has on OpenStack is described here: https://bugzilla.redhat.com/show_bug.cgi?id=835466#c9 Rich.

12 years, 12 months

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NOTICE: Data corruption bug when writing to qcow2 files

by Richard W.M. Jones

As you might have seen for the past 3 days I've been tackling a nasty data corruption bug[1][2]. The bug occurs when ALL of the following conditions are true: (a) You are using a qcow2 image file. (b) You are writing out data to the image file using libguestfs or a libguestfs-using tool like guestfish or virt-resize. (c) The data is not being written to a filesystem (to files or directories) but is being written directly to a block device within libguestfs, eg. updates to the partition table or writes directly to /dev/sdaX. (d) You are using qemu < 1.1.0. When the guestfs handle is closed, the data might not be written to the qcow2 image file. This data loss, if it happens, is silent. This peculiar combination of factors happened to occur in the virt-resize test program[3], and this was where I first spotted it[4] although at first it didn't look like a data corruption bug at all. After analysis I found that there are four separate bugs involved: (i) qemu had a bug where it would segfault when you sent it a SIGTERM signal. It turned out that where qemu was writing to a qcow2 file, and the qcow2 writeback cache is enabled [NB: cache=none enables this cache], and write requests were in flight at the point when the SIGERM is received, it would crash. ** This bug has been fixed in qemu/qemu-kvm >= 1.1.0. It is highly ** recommended that you immediately upgrade to this version, not ** just for libguestfs but for all usage. (ii) The Linux kernel sync(2) system call doesn't issue a write barrier for dirty blocks that are written to a block device directly, only for mounted filesystems. This bug will probably be fixed if the following patch goes upstream: https://lkml.org/lkml/2012/7/3/277 (iii) libguestfs was issuing sync(2) in the expectation that it flushed everything. The implication for libguestfs is that the qemu cache still contains data at the point when we kill qemu. Bugs (i) and (ii) unexpectedly interact. (iv) libguestfs didn't check the return value for waitpid(2) so it didn't know that qemu was segfaulting, so this loss of data was silently ignored. Bug (i) can be fixed by updating to qemu 1.1.0. Unfortunately we do not know which precise commit between 1.0 and 1.1.0 fixed the bug, and doing a git bisect is difficult because the data corruption bug is very hard to reproduce reliably. Bugs (iii) and (iv) will be fixed by forthcoming patches to libguestfs >= 1.19.16 which will be backported to 1.16 and 1.18 branches. Note that this requires a new API, guestfs_shutdown[5]. If your program wants to handle write errors correctly it will need to use this new API, otherwise an error will be printed and ignored. All libguestfs tools that modify disk images have been updated to use the new API. Hans de Goede is currently updating Fedora to qemu-kvm 1.1.0. Versions of libguestfs which contain fixes will be announced separately. It is likely that these versions will *require* qemu >= 1.1.0, so effectively our baseline version of qemu has just increased from 1.0 to 1.1.0, and this change is noted in the README file. (Thanks to Kevin Wolf, Paolo Bonzini, Avi Kivity, Padraig Brady for invaluable help.) Rich. [1] https://www.redhat.com/archives/libguestfs/2012-July/msg00005.html [2] https://www.redhat.com/archives/libguestfs/2012-July/msg00008.html [3] https://github.com/libguestfs/libguestfs/blob/cb24ceedd8a8ef7da71cfcce6db... [4] https://bugzilla.redhat.com/show_bug.cgi?id=836710 [5] https://www.redhat.com/archives/libguestfs/2012-July/msg00014.html -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://et.redhat.com/~rjones/virt-df/

12 years, 12 months

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[PATCH 0/7 v2] Fix and workaround for qcow2 issues in qemu causing data corruption.

by Richard W.M. Jones

https://bugzilla.redhat.com/show_bug.cgi?id=836710 https://bugzilla.redhat.com/show_bug.cgi?id=836913 There are at least two related bugs going on: (1) Linux sync(2) system call doesn't send a write barrier to the disk, so in effect it doesn't force the hard disk to flush its cache. libguestfs used sync(2) to force changes to disk. We didn't expect that qemu was caching anything because we used 'cache=none' for all writable disks, but it turns out that qemu creates a writeback cache anyway when you do this (you need to use 'cache=directsync' when you don't want a cache at all). (2) qemu's qcow2 disk cache code is buggy. If there are I/Os in flight when qemu shuts down, then qemu segfaults or assert fails. This can result in unwritten data. Unfortunately libguestfs ignored the result of waitpid(2) so we didn't see this problem happening. Patch 1/7 fixes the first problem by issuing fsync(2) on each whole block device when we sync. Patches 2/7 - 7/7 are needed to fix the second problem. We add a new API (guestfs_shutdown) so that we can actually catch the case where qemu is segfaulting instead of just ignoring it. Since qemu itself isn't likely to be fixed any time soon, patch 7/7 adds a crude but effective workaround to virt-resize. Rich.

13 years

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unexpected end of file mounting vdi image

by B. M.

I am unable to mount a vdi image with virt-df. I get this error in the libvirt log. 2012-06-24 02:37:22.484+0000: 2364: error : qemuMonitorIO:583 : internal error End of file from monitor I don't really know from this output what the problem is and some assistance would be appreciated. I'm attaching the output of the libguest test program. Brian ____________________________________________________________ GET FREE SMILEYS FOR YOUR IM & EMAIL - Learn more at http://www.inbox.com/smileys Works with AIM®, MSN® Messenger, Yahoo!® Messenger, ICQ®, Google Talk™ and most webmails

13 years

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[PATCH POSSIBLE DATA CORRUPTOR] daemon: Run fsync on block devices after sync (RHBZ#836710).

by Richard W.M. Jones

This is by no means a complete fix for RHBZ#836710, because I haven't completely got to the bottom of this bug yet, but it is an improvement. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones New in Fedora 11: Fedora Windows cross-compiler. Compile Windows programs, test, and build Windows installers. Over 70 libraries supprt'd http://fedoraproject.org/wiki/MinGW http://www.annexia.org/fedora_mingw

13 years

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New tool proposal

by Wanlong Gao

Hi Rich, We just talked about a new tool virt-diff which can diff files between two guest, suggested by Kamezawa Hiroyuki <kamezawa.hiroyu(a)jp.fujitsu.com> things like below, # virt-diff -Nru --guest=Guest0 --guest=Guest1 /etc/hosts Any thought about this? Another question, what do you think about implement iptables, firewall, yum, rpm, etc into libguestfs? Thanks, Wanlong Gao

13 years

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Libguestfs July 2012