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Re: [libvirt] NFS over RDMA small block DIRECT_IO bug



On Thu, 2012-09-06 at 12:14 +0200, Andrew Holway wrote:
> 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 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 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!!

A couple of RDMA folks are looking into why this is happening. I'm
hoping they will get back to me soon.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond Myklebust netapp com
www.netapp.com



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