Re: [Qemu-devel] callout to *file in bdrv_co_get_block_status

From: Fam Zheng <famz@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Lieven <pl@kamp.de>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	qemu block <qemu-block@nongnu.org>
Subject: Re: [Qemu-devel] callout to *file in bdrv_co_get_block_status
Date: Mon, 20 Mar 2017 19:49:26 +0800	[thread overview]
Message-ID: <20170320114926.GB17020@lemon.lan> (raw)
In-Reply-To: <37879546-cf6e-01fa-adc6-c777e14eab0e@redhat.com>

On Mon, 03/20 12:21, Paolo Bonzini wrote:
> 
> 
> On 20/03/2017 03:46, Fam Zheng wrote:
> > On Fri, 03/17 12:20, Peter Lieven wrote:
> >> Am 17.03.2017 um 12:16 schrieb Paolo Bonzini:
> >>>
> >>> On 17/03/2017 12:11, Peter Lieven wrote:
> >>>>>> like VMDK or QCOW2 shouldn't we trust the information from the l2 tables in the VMDK or QCOW2?
> >>>>> It provides additional information, for example it works better with
> >>>>> prealloc=metadata.
> >>>> Okay, understood. Can you imagine of a away to conditionally avoid this second callout? In my case we have an additional
> >>>> lseek for each cluster. For a 20GB file this are approx. 327k calls to lseek. And if the file has no preallocated metadata
> >>>> it will likely not improve anything. And even if the metadata is prealloced what is the allocation status of the clusters?
> >>> If the metadata is preallocated, cluster will (or should) show up as
> >>> zero, speeding up the copy.
> >>
> >> Okay, in this case the second call out to *file will not happen. It only happens if the metadata says it contains data.
> >> So where does it actually help?
> >>
> >> The condition is: (ret & BDRV_BLOCK_DATA) && !(ret & BDRV_BLOCK_ZERO) && (ret & BDRV_BLOCK_OFFSET_VALID))
> >>
> >> So from my view it can only have any effect if the metadata returns BDRV_BLOCK_DATA, but the protocol driver returns
> >> BDRV_BLOCK_ZERO.
> >>
> >> This can only happen if I partially write to a cluster, or am I wrong here?
> > 
> > I think you have a point. The metadata should have said BDRV_BLOCK_ZERO if
> > protocol would say BDRV_BLOCK_ZERO - there is no reason the format driver cannot
> > know.
> 
> That's true of qcow2, but many formats (including raw :)) don't know
> about BDRV_BLOCK_ZERO.

Raw is a little special, it could have forwarded the call to *file in its
BlockDriver callback. Most formats with metadata stores zero/nonzero information
in L1/L2 tables. For qcow2 and VMDK I think it's okay to just trust meta data on
zero/nonzero.

Fam