Re: [Qemu-devel] Strange problems with lseek in qemu-img map

[Stefan Hajnoczi <stefanha@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 2052 bytes --]

On Tue, Jun 02, 2015 at 02:54:17PM +0200, David Weber wrote:
> 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

These systems have the same kernel but for some reason O3-3 completes
quickly while Dinah takes a long time in lseek(fd, offset, SEEK_DATA).
It looks like the file is empty (the syscall keeps returning ENXIO
because there are no allocated blocks in the file where qemu-img
probes).

> 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)
> 
> 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.

"it" == qemu-img map hangs or takes a very long time?

Can you post a shell script that reproduces this with a ramdisk?  That
seems like the easiest way to get people debugging it.

Stefan

[-- Attachment #2: Type: application/pgp-signature, Size: 473 bytes --]