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Date: Thu, 4 Jun 2015 09:19:20 +0800
From: Wen Congyang <wency@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] Strange problems with lseek in qemu-img map
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To: David Weber <wb@munzinger.de>, qemu-devel@nongnu.org

On 06/02/2015 08:54 PM, David Weber wrote:
> Hello,
> 
> I'm currently evaluating to switch our virtualization servers to a newer OS. 
> This includes a switch from qemu 1.7 to 2.2 or 2.3. 
> Our system heavily relies on big sparse images and drive_mirror. While 
> testing, I experienced some problems with that combination.
> The strange thing is, that everything works flawlessly on my workstation but 
> fails on my servers.
> 
> Testcase:
> # qemu-img create test 500G
> # time qemu-img map test
> 
> Systems:
> O3-3: Kubuntu 15.04 Workstation with stock-kernel 3.19.0-18-generic and stock 
> qemu 2.2.0
> Dinah: Ubuntu Server 15.04 with stock-kernel 3.19.0-18-generic and stock qemu 
> 2.2.0
> 
> Result on O3-3:
> root@o3-3:~# qemu-img create test 500G
> Formatting 'test', fmt=raw size=536870912000 
> root@o3-3:~# time qemu-img map test
> Offset          Length          Mapped to       File
> 
> real    0m0.049s
> user    0m0.048s
> sys     0m0.000s
> 
> Result on dinah:
> root@dinah:~# qemu-img create test 500G
> Formatting 'test', fmt=raw size=536870912000 
> root@dinah:~# time qemu-img map test
> Offset          Length          Mapped to       File
> ^C
> 
> real    0m41.862s
> user    0m0.004s
> sys     0m0.068s
> (Stopped with ^C)

Do you use the same filesystem?

Thanks
Wen Congyang

> 
> Strace on O3-3:
> https://gist.github.com/anonymous/f221035e9176f7c71c74
> 
> Strace on dinah:
> https://gist.github.com/anonymous/40b42888a65478c90b32
> 
> A git bisect between 1.7 and master revealed 
> 7c15903789953ead14a417882657d52dc0c19a24 "block/raw-posix: use seek_hole ahead 
> of fiemap" as bad but this is not the real problem.
> I also tried to switch from btrfs to ext4 but it didn't change anything.
> 
> At this point, I was pretty sure that was just stupit and missing something 
> trivial.
> I then startet a fedora 22 live system and I saw the same problem. It happens 
> on both the ramdisk and a ext4 filesystem.
> 
> Any ideas on this? I'm pretty much stuck at this point. Please ask if you need 
> more information.
> 
> Cheers,
> David
> 
> 
>