Hello, > I agree with Robert, the QEMU command-line used to launch these guests > would be useful (on host: ps aux | grep kvm). Have you explicitly set > a -drive cache= mode (like "none", "writeback", or "writethrough")? here is the qemu command line we are using (or which libvirt generates): /usr/bin/kvm -S -M pc-0.12 -enable-kvm -m 512 -smp 1,sockets=1,cores=1,threads=1 -name vm-933 -uuid 0d737610-e59b-012d-f453-32287f7402ab -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/vm-933.monitor,server,nowait -mon chardev=monitor,mode=readline -rtc base=utc -boot nc -drive file=/dev/vg0/934,if=none,id=drive-ide0-0-0,boot=on,format=raw,cache=writeback -device ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0 -device rtl8139,vlan=0,id=net0,mac=00:1c:14:01:03:67,bus=pci.0,addr=0x3 -net tap,fd=23,vlan=0,name=hostnet0 -chardev pty,id=serial0 -device isa-serial,chardev=serial0 -usb -vga cirrus -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 We are explicit using "writeback" in cache settings. > The first backtrace is odd. You are using logical volumes for the > guest but the backtrace shows kjournald is blocked. I believe logical > volumes should not directly affect kjournald at all (they don't use > journalling). Perhaps this is a deadlock. The dmesg output was just an example. Most of the time I can see the tasks "kjournald" and "flush". I recently saw "kvm","kthreadd","rsyslogd" and others in such outputs. I thought that sometimes /proc/sys/vm/dirty_ratio gets exceeded and so all processes are blocked for writes to the cache (kvm processes too). Could this be the case? I set "dirty_background_ratio" to 5 to constantly flush the cache to the disk, but this did not help. > About the "flush-251:0:505" hang, please cat /proc/partitions on the > host to see which block device has major number 251 and minor number 0 > is. This is our logical volume "root" partition of the physical host. > The fact that your host is having problems suggests the issue is not > in qemu-kvm (it's just a userspace process). Are you sure disk I/O is > working under load on this machine without KVM? I do not think that kvm generates this issue (as you said it is a normal user space process). I thought that perhaps somebody knows how to handle this situation, because the kvm developers have much more experience with kvm than I do. Perhaps there are some tuning tips for this or anybody knows why only OpenSuse sets the filesystem read only if there are disk timeouts in the guest? This behavior appeared on almost all hosts (>20) so I can eliminate a single machine HW failure. Thank you for your help... Best Regards Sebastian Am Donnerstag, den 09.12.2010, 10:30 +0000 schrieb Stefan Hajnoczi: > On Thu, Dec 9, 2010 at 8:10 AM, Sebastian Nickel - Hetzner Online AG > <sebastian.nickel@xxxxxxxxxx> wrote: > > Hello, > > we have got some issues with I/O in our kvm environment. We are using > > kernel version 2.6.32 (Ubuntu 10.04 LTS) to virtualise our hosts and we > > are using ksm, too. Recently we noticed that sometimes the guest systems > > (mainly OpenSuse guest systems) suddenly have a read only filesystem. > > After some inspection we found out that the guest system generates some > > ata errors due to timeouts (mostly in "flush cache" situations). On the > > physical host there are always the same kernel messages when this > > happens: > > > > """ > > [1508127.195469] INFO: task kjournald:497 blocked for more than 120 > > seconds. > > [1508127.212828] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" > > disables this message. > > [1508127.246841] kjournald D 00000000ffffffff 0 497 2 > > 0x00000000 > > [1508127.246848] ffff88062128dba0 0000000000000046 0000000000015bc0 > > 0000000000015bc0 > > [1508127.246855] ffff880621089ab0 ffff88062128dfd8 0000000000015bc0 > > ffff8806210896f0 > > [1508127.246862] 0000000000015bc0 ffff88062128dfd8 0000000000015bc0 > > ffff880621089ab0 > > [1508127.246868] Call Trace: > > [1508127.246880] [<ffffffff8116e500>] ? sync_buffer+0x0/0x50 > > [1508127.246889] [<ffffffff81557d87>] io_schedule+0x47/0x70 > > [1508127.246893] [<ffffffff8116e545>] sync_buffer+0x45/0x50 > > [1508127.246897] [<ffffffff8155825a>] __wait_on_bit_lock+0x5a/0xc0 > > [1508127.246901] [<ffffffff8116e500>] ? sync_buffer+0x0/0x50 > > [1508127.246905] [<ffffffff81558338>] out_of_line_wait_on_bit_lock > > +0x78/0x90 > > [1508127.246911] [<ffffffff810850d0>] ? wake_bit_function+0x0/0x40 > > [1508127.246915] [<ffffffff8116e6c6>] __lock_buffer+0x36/0x40 > > [1508127.246920] [<ffffffff81213d11>] journal_submit_data_buffers > > +0x311/0x320 > > [1508127.246924] [<ffffffff81213ff2>] journal_commit_transaction > > +0x2d2/0xe40 > > [1508127.246931] [<ffffffff810397a9>] ? default_spin_lock_flags > > +0x9/0x10 > > [1508127.246935] [<ffffffff81076c7c>] ? lock_timer_base+0x3c/0x70 > > [1508127.246939] [<ffffffff81077719>] ? try_to_del_timer_sync+0x79/0xd0 > > [1508127.246943] [<ffffffff81217f0d>] kjournald+0xed/0x250 > > [1508127.246947] [<ffffffff81085090>] ? autoremove_wake_function > > +0x0/0x40 > > [1508127.246951] [<ffffffff81217e20>] ? kjournald+0x0/0x250 > > [1508127.246954] [<ffffffff81084d16>] kthread+0x96/0xa0 > > [1508127.246959] [<ffffffff810141ea>] child_rip+0xa/0x20 > > [1508127.246962] [<ffffffff81084c80>] ? kthread+0x0/0xa0 > > [1508127.246966] [<ffffffff810141e0>] ? child_rip+0x0/0x20 > > [1508127.246969] INFO: task flush-251:0:505 blocked for more than 120 > > seconds. > > [1508127.264076] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" > > disables this message. > > [1508127.298343] flush-251:0 D ffffffff810f4370 0 505 2 > > 0x00000000 > > [1508127.298349] ffff880621fdba30 0000000000000046 0000000000015bc0 > > 0000000000015bc0 > > [1508127.298354] ffff88062108b1a0 ffff880621fdbfd8 0000000000015bc0 > > ffff88062108ade0 > > [1508127.298358] 0000000000015bc0 ffff880621fdbfd8 0000000000015bc0 > > ffff88062108b1a0 > > [1508127.298362] Call Trace: > > [1508127.298370] [<ffffffff810f4370>] ? sync_page+0x0/0x50 > > [1508127.298375] [<ffffffff81557d87>] io_schedule+0x47/0x70 > > [1508127.298379] [<ffffffff810f43ad>] sync_page+0x3d/0x50 > > [1508127.298383] [<ffffffff8155825a>] __wait_on_bit_lock+0x5a/0xc0 > > [1508127.298391] [<ffffffff810f4347>] __lock_page+0x67/0x70 > > [1508127.298395] [<ffffffff810850d0>] ? wake_bit_function+0x0/0x40 > > [1508127.298402] [<ffffffff810f4487>] ? unlock_page+0x27/0x30 > > [1508127.298410] [<ffffffff810fd9dd>] write_cache_pages+0x3bd/0x4d0 > > [1508127.298417] [<ffffffff810fc670>] ? __writepage+0x0/0x40 > > [1508127.298425] [<ffffffff810fdb14>] generic_writepages+0x24/0x30 > > [1508127.298432] [<ffffffff810fdb55>] do_writepages+0x35/0x40 > > [1508127.298439] [<ffffffff811668a6>] writeback_single_inode+0xf6/0x3d0 > > [1508127.298449] [<ffffffff812b81d6>] ? rb_erase+0xd6/0x160 > > [1508127.298455] [<ffffffff8116750e>] writeback_inodes_wb+0x40e/0x5e0 > > [1508127.298462] [<ffffffff811677ea>] wb_writeback+0x10a/0x1d0 > > [1508127.298469] [<ffffffff81077719>] ? try_to_del_timer_sync+0x79/0xd0 > > [1508127.298477] [<ffffffff8155803d>] ? schedule_timeout+0x19d/0x300 > > [1508127.298485] [<ffffffff81167b1c>] wb_do_writeback+0x18c/0x1a0 > > [1508127.298493] [<ffffffff81167b83>] bdi_writeback_task+0x53/0xe0 > > [1508127.298503] [<ffffffff8110f546>] bdi_start_fn+0x86/0x100 > > [1508127.298510] [<ffffffff8110f4c0>] ? bdi_start_fn+0x0/0x100 > > [1508127.298518] [<ffffffff81084d16>] kthread+0x96/0xa0 > > [1508127.298526] [<ffffffff810141ea>] child_rip+0xa/0x20 > > [1508127.298533] [<ffffffff81084c80>] ? kthread+0x0/0xa0 > > [1508127.298541] [<ffffffff810141e0>] ? child_rip+0x0/0x20 > > ... some more messages like those before... > > """ > > Sometimes the message "INFO: task kvm blocked for more than 120 seconds" > > appears, too. I thought the error happens in cache writeback situations > > so I started to adjust "/proc/sys/vm/dirty_background_ratio" to 5 and > > "/proc/sys/vm/dirty_ratio" to 40. I thought this will write continously > > smaller parts of cached memory to HDD (more often, but smaller chunks). > > This did not help. There are still "readonly" filesystems in the guest > > systems. Does anybody has some tips to regulate I/O on linux systems or > > to stop those "readonly" filesystems? > [...] > > We are using logical volumes as virtual guest HDDs. The volume group is > > on top of a mdraid device. > > I agree with Robert, the QEMU command-line used to launch these guests > would be useful (on host: ps aux | grep kvm). Have you explicitly set > a -drive cache= mode (like "none", "writeback", or "writethrough")? > > The first backtrace is odd. You are using logical volumes for the > guest but the backtrace shows kjournald is blocked. I believe logical > volumes should not directly affect kjournald at all (they don't use > journalling). Perhaps this is a deadlock. > > About the "flush-251:0:505" hang, please cat /proc/partitions on the > host to see which block device has major number 251 and minor number 0 > is. > > The fact that your host is having problems suggests the issue is not > in qemu-kvm (it's just a userspace process). Are you sure disk I/O is > working under load on this machine without KVM? > > Stefan -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
