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Re: COMPARE_AND_WRITE support for rbd driver

From: Yaowei Bai
Subject: Re: COMPARE_AND_WRITE support for rbd driver
Date: Fri, 20 Sep 2019 23:26:28 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Sep 20, 2019 at 01:22:02PM +0200, Paolo Bonzini wrote:
> On 19/09/19 07:36, Yaowei Bai wrote:
> > 
> > Hey guys,
> > 
> > I noticed that COMPARE_AND_WRITE had been supported by CEPH/librbd since
> > v12.1.1. And in my company, we use this COMPARE_AND_WRITE support in
> > CEPH with the ISCSI protocol. More precisely, we use tgt and CEPH with this
> > COMPARE_AND_WRITE support as the SCSI target and export it to the remote
> > hosts. And then VMs on remote hosts can use these SCSI targets through ISCSI
> > initiator support in QEMU directly or as local SCSI disks. But 
> > unfortunately,
> > there're some issues with this tgt case. So i think maybe we could also add 
> > this
> > COMPARE_AND_WRITE support into the rbd driver in QEMU so we can leave the
> > ISCSI/tgt alone and use this COMPARE_AND_WRITE support with the
> > scsi-disk <--> virtio-scsi <--> rbd driver path. This can also apply to
> > the WRITESAME support in CEPH/librbd.
> > 
> > So is it suitable for doing this?
> Yes, it would be suitable.  In a nutshell you would have to add support
> for COMPARE_AND_WRITE to block/io.c (calling into a new BlockDriver
> function pointer), block/rbd.c and hw/scsi/scsi-disk.c.

OK, i'll try to implement this and send it out for review. Thanks Paolo.

> Paolo

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