[Qemu-devel] Re-2: Strange problems with lseek in qemu-img map

[David Weber]

Thanks a lot for your reply and sorry for my double post out of impatientness.

> > 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.

Running everything on tmpfs instead is fast
mkdir -p /mnt/tmp
mount -t tmpfs -o size=1G none /mnt/tmp
cd /mnt/tmp
qemu-img create test 500G
time qemu-img map test

I did run a few more tests on different systems and made an interessting 
observation.
lseek is damn slow if the blockdevice has an I/O size of 512 bytes (like the 
raid controller of my server or a ramdisk) and runs fast with 4096 bytes (like 
the ssd of my workstation or tmpfs).
I confirmned this also with with a few other drives but it still could be 
coincidence...

You can check the I/O size by running 
fdisk <dev>
p

Cheers,
David

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