[libvirt] NFS over RDMA small block DIRECT_IO bug

[Andrew Holway]

On Sep 5, 2012, at 4:02 PM, Avi Kivity wrote:

> On 09/04/2012 03:04 PM, Myklebust, Trond wrote:
>> On Tue, 2012-09-04 at 11:31 +0200, Andrew Holway wrote:
>>> Hello.
>>> 
>>> # Avi Kivity avi(a)redhat recommended I copy kvm in on this. It would also seem relevent to libvirt. #
>>> 
>>> I have a Centos 6.2 server and Centos 6.2 client.
>>> 
>>> [root at store ~]# cat /etc/exports 
>>> /dev/shm				10.149.0.0/16(rw,fsid=1,no_root_squash,insecure)    (I have tried with non tempfs targets also)
>>> 
>>> 
>>> [root at node001 ~]# cat /etc/fstab 
>>> store.ibnet:/dev/shm             /mnt                 nfs          rdma,port=2050,defaults 0 0
>>> 
>>> 
>>> I wrote a little for loop one liner that dd'd the centos net install image to a file called 'hello' then checksummed that file. Each iteration uses a different block size.
>>> 
>>> Non DIRECT_IO seems to work fine. DIRECT_IO with 512byte, 1K and 2K block sizes get corrupted.
>> 
>> 
>> That is expected behaviour. DIRECT_IO over RDMA needs to be page aligned
>> so that it can use the more efficient RDMA READ and RDMA WRITE memory
>> semantics (instead of the SEND/RECEIVE channel semantics).
> 
> Shouldn't subpage requests fail then?  O_DIRECT block requests fail for
> subsector writes, instead of corrupting your data.

But silent data corruption is so much fun!!