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Date: Wed, 9 Mar 2016 16:33:18 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 06/16] block: add BDS field to count
	in-flight requests
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: stefanha@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org

On Wed, 03/09 09:22, Paolo Bonzini wrote:
> 
> 
> On 09/03/2016 09:00, Fam Zheng wrote:
> >> > On 09/03/2016 04:35, Fam Zheng wrote:
> >>>>> > >> >  enum BdrvTrackedRequestType {
> >>>>> > >> >      BDRV_TRACKED_READ,
> >>>>> > >> >      BDRV_TRACKED_WRITE,
> >>>>> > >> > -    BDRV_TRACKED_FLUSH,
> >>>>> > >> > -    BDRV_TRACKED_IOCTL,
> >>>>> > >> >      BDRV_TRACKED_DISCARD,
> >>> > > Okay, so flush and ioctl are not needed, but why is discard different?
> >> > 
> >> > Discard can modify the contents of the device, so I think it's safer to
> >> > serialize it against RMW and copy-on-read operations.
> > Okay, that makes sense, but ioctl like SG_IO can also modify content, no?
> 
> If you use SG_IO you shouldn't use RMW (because scsi_read_complete traps
> READ CAPACITY and sets the host block size as the guest block size) or
> copy-on-read (because raw has no backing file).
> 
> Besides, BDRV_TRACKED_IOCTL didn't include sector_num/nr_sectors
> operations so it didn't provide serialization.
> 

Yes, that's right. Thanks.

Fam