Re: Re-2: Strange problems with lseek in qemu-img map

Stefan Hajnoczi
2015-06-05T14:05:15Z

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On Wed, Jun 03, 2015 at 03:09:40PM +0000, David Weber wrote:
> > > 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?
> I never waited for it to complete but I guess it just takes very long.
>
> >
> > Can you post a shell script that reproduces this with a ramdisk? That
> > seems like the easiest way to get people debugging it.
>
> You can use the following commands.
>
> mkfs.ext4 /dev/ram0
> mkdir -p /mnt/tmp
> mount /dev/ram0 /mnt/tmp
> cd /mnt/tmp
> qemu-img create test 500G
> time qemu-img map test
>
> This takes foreover on all my systems.

I experience the same thing on Linux 4.1.0-rc5 with ext4 (4 KB file
system block size, 512B device sector size).

The lseek() calls take *seconds* to complete on a completely empty
(sparse) 500G file.

Here is the perf-top(1) output:

34.08% [kernel] [k] _raw_read_lock
21.11% [kernel] [k] ext4_es_lookup_extent
20.67% [kernel] [k] ext4_es_find_delayed_extent_range
8.68% [kernel] [k] ext4_map_blocks
4.99% [kernel] [k] ext4_llseek
1.90% [kernel] [k] rb_next
0.14% [kernel] [k] __fget

Yikes!

The syscall causing this is just:

lseek(7, 0, SEEK_DATA)

Lukas: I've CCed you because you have helped with ext4 issues in the
past. Not sure if you have time or if this is your area.

Stefan

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