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Re: [Qemu-block] [Qemu-devel] Migration sometimes fails with IDE and Qem


From: Peter Lieven
Subject: Re: [Qemu-block] [Qemu-devel] Migration sometimes fails with IDE and Qemu 2.2.1
Date: Thu, 09 Apr 2015 14:46:40 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

Am 07.04.2015 um 21:13 schrieb John Snow:


On 04/07/2015 03:02 PM, Peter Lieven wrote:
Am 07.04.2015 um 20:56 schrieb John Snow:


On 04/07/2015 02:44 PM, Peter Lieven wrote:
Am 07.04.2015 um 17:29 schrieb Dr. David Alan Gilbert:
* Peter Lieven (address@hidden) wrote:
Hi David,

Am 07.04.2015 um 10:43 schrieb Dr. David Alan Gilbert:
Any particular workload or reproducer?
Workload is almost zero. I try to figure out if there is a way to trigger it.

Maybe playing a role: Machine type is -M pc1.2 and we set -kvmclock as
CPU flag since kvmclock seemed to be quite buggy in 2.6.16...

Exact cmdline is:
/usr/bin/qemu-2.2.1 -enable-kvm -M pc-1.2 -nodefaults -netdev type=tap,id=guest2,script=no,downscript=no,ifname=tap2 -device e1000,netdev=guest2,mac=52:54:00:ff:00:65 -drive format=raw,file=iscsi://172.21.200.53/iqn.2001-05.com.equallogic:4-52aed6-88a7e99a4-d9e00040fdc509a3-XXX-hd0/0,if=ide,cache=writeback,aio=native -serial null -parallel null -m 1024 -smp 2,sockets=1,cores=2,threads=1 -monitor tcp:0:4003,server,nowait -vnc :3 -qmp tcp:0:3003,server,nowait -name 'XXX' -boot order=c,once=dc,menu=off -drive index=2,media=cdrom,if=ide,cache=unsafe,aio=native,readonly=on -k de -incoming tcp:0:5003 -pidfile /var/run/qemu/vm-146.pid -mem-path /hugepages -mem-prealloc -rtc base=utc -usb -usbdevice tablet -no-hpet -vga cirrus -cpu qemu64,-kvmclock

Exact kernel is:
2.6.16.46-0.12-smp (i think this is SLES10 or sth.)

The machine does not hang. It seems just I/O is hanging. So you can type at the 
console or ping the system, but no longer login.

Thank you,
Peter
Interesting observation: Migrating the vServer again seems to fix to problem 
(at least in one case I could test just now).

2.6.8-24-smp is also affected.
How often does it fail - you say 'sometimes' - is it a 1/10 or a 1/1000 ?
Its more often than 1/10 I would say.
OK, that's not too bad - it's the 1/1000 that are really nasty to find.
In your setup, how easy would it be for you to try :
      with either 2.1 or current head?
      with a newer machine-type?
      without the cdrom?

Its all possible. I can clone the system and try everything on my test systems. 
I hope
it reproduces there.

Has the cdrom the power of taking down the bus?

Peter


I don't know if CDROM could stall the entire bus, but I suspect the reason for asking is this: dgilbert and I had tracked down a problem previously where during migration, outstanding requests being handled by the ATAPI code can get lost during migration if, for instance, the user has only prepared the command (via bmdma) but has not yet written to the register to activate the command yet.

That sounds like it could be related.


So if something like this happens:

- User writes to the ATA registers to program a command
- Migration occurs
- User writes to the BMDMA register to initiate the command

We can lose some of the state and data of the request. David had checked in a 
workaround for at least ATAPI that simply coaxes the guest OS into trying the 
command again to unstick it.

Do you have the commit for me?


http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg01109.html


I think we determined last time that we couldn't fix this problem without changing the migration format, so we opted not to do it for 2.3. We had also only noticed it with ATAPI drives, not HDDs, so a proper fix got kicked down the road since we thought the workaround was sufficient.

Maybe normally we use virtio nowadays and maybe the new kernel implementation 
(libata /dev/sdX) can't get locked? What I do not understand is how a second 
migration can unlock from this state?


IIRC our success rate with reproducing it was something on the order of 1/50, 
too.

If you can reproduce it without a CDROM but using the BMDMA interface, that's a 
good data point. If you can't reproduce it using the ISA interface, that's a 
phenomenal data point and implicates BMDMA pretty heavily.

To be 100% sure we are talking about the same? How would I use the ISA and how 
would I use the BMDMA interface?

Thanks,
Peter


BMDMA is the PCI HBA for IDE, I think it's the default for most machines that 
aren't using the AHCI HBA.

To get ISA, try launching with the machine "isapc" which will force it, or add the device 
manually, it's named "isa-ide".
The BMDMA PCI device is just named "ide".

Unfortunately, the BIOS can't boot if I specify device isa-ide.

Peter



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