Re: [Qemu-devel] scsi-hd with discard_granularity and unmap results in Aborted Commands

[Paolo Bonzini]

Il 19/11/2012 10:36, Stefan Priebe - Profihost AG ha scritto:
> Hi Paolo,
> 
> Am 19.11.2012 09:10, schrieb Paolo Bonzini:
>>> I'm sorry the discard requests aren't failing. Qemu / Block driver
>>> starts to cancel a bunch of requests.
>>
>> That is being done in the kernel (the guest, I think) because the UNMAPs
>> are taking too long.
> 
> That makes sense. RBD handles discards as buffered I/O. When i do an
> mkfs.xfs on a 30GB device i see around 900 pending discard requests. rbd
> finishes them all before reporting success.
> 
> If it is correct that a 3.6.7 kernel sends as many discard requests i
> only the a solution in using unbuffered I/O for discards.
> 
> Do you know what is the correct way?

I think the correct fix is to serialize them in the kernel.

Paolo