Re: raw disks no longer work in latest kvm (kvm-88 was fine)

[Antoine Martin <antoine@nagafix.co.uk>]

On 05/23/2010 10:12 PM, Avi Kivity wrote:
> On 05/23/2010 05:43 PM, Antoine Martin wrote:
>>>> Description of the problem:
>>>> A guest tries to mount a large raw partition (1.5TB /dev/sda9 in my 
>>>> case), this fails with pread enabled, works with it disabled.
>>>
>>> Did you mean: preadv?
>>
>> Yes, here's what makes it work ok (as suggested by Christoph earlier 
>> in the thread) in posix-aio-compat.c:
>> #undef CONFIG_PREADV
>> #ifdef CONFIG_PREADV
>> static int preadv_present = 1;
>> #else
>> static int preadv_present = 0;
>> #endif
>
> When preadv_present=1, does strace -fF show preadv being called?
There were some pread() but no preadv() in the strace BTW.
> If so, the kernel's preadv() is broken.
>
> If not, glibc's preadv() emulation is broken.
OK, I found what's causing it: chroot
I was testing all this in a chroot, I always do and I didn't think of 
mentioning it, sorry about that.
Running it non-chrooted works in all cases, including aio!=native

Why does it work in a chroot for the other options (aio=native, if=ide, 
etc) but not for aio!=native??
Looks like I am misunderstanding the semantics of chroot...

>>>
>>> Does aio=native help?
>> It does indeed!
>
> That's recommended anyway on raw partitions with cache=off.
>
What about non-raw partitions (or/and with cache=on)?

Antoine