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Old 08-16-2010, 06:33 PM
Christoph Hellwig
 
Default virtio_blk: implement REQ_FLUSH/FUA support

On Mon, Aug 16, 2010 at 06:52:00PM +0200, Tejun Heo wrote:
> From: Tejun Heo <tj@kernle.org>
>
> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
> indicate the support for FUA.

I'm not sure it's worth it. The pure REQ_FLUSH path works not and is
well tested with kvm/qemu. We can still easily add a FUA bit, and
even a pre-flush bit if the protocol roundtrips matter in real life
benchmarking.

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Old 08-17-2010, 01:16 AM
Rusty Russell
 
Default virtio_blk: implement REQ_FLUSH/FUA support

On Tue, 17 Aug 2010 02:22:00 am Tejun Heo wrote:
> From: Tejun Heo <tj@kernle.org>
>
> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
> indicate the support for FUA.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Cc: Michael S. Tsirkin <mst@redhat.com>

Acked-by: Rusty Russell <rusty@rustcorp.com.au>

And also for the lguest-specific patch.

Thanks!
Rusty.

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Old 08-17-2010, 08:17 AM
Tejun Heo
 
Default virtio_blk: implement REQ_FLUSH/FUA support

Hello,

On 08/16/2010 08:33 PM, Christoph Hellwig wrote:
> On Mon, Aug 16, 2010 at 06:52:00PM +0200, Tejun Heo wrote:
>> From: Tejun Heo <tj@kernle.org>
>>
>> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
>> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
>> indicate the support for FUA.
>
> I'm not sure it's worth it. The pure REQ_FLUSH path works not and is
> well tested with kvm/qemu. We can still easily add a FUA bit, and
> even a pre-flush bit if the protocol roundtrips matter in real life
> benchmarking.

Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
should be cheaper than FLUSH.

Thanks.

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Old 08-17-2010, 08:18 AM
Tejun Heo
 
Default virtio_blk: implement REQ_FLUSH/FUA support

On 08/17/2010 03:16 AM, Rusty Russell wrote:
> On Tue, 17 Aug 2010 02:22:00 am Tejun Heo wrote:
>> From: Tejun Heo <tj@kernle.org>
>>
>> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
>> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
>> indicate the support for FUA.
>>
>> Signed-off-by: Tejun Heo <tj@kernel.org>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>
> Acked-by: Rusty Russell <rusty@rustcorp.com.au>
>
> And also for the lguest-specific patch.

Thanks. Acked-by's added.

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Old 08-17-2010, 01:23 PM
Christoph Hellwig
 
Default virtio_blk: implement REQ_FLUSH/FUA support

On Tue, Aug 17, 2010 at 10:17:15AM +0200, Tejun Heo wrote:
> >> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
> >> support instead. A new feature flag VIRTIO_BLK_F_FUA is added to
> >> indicate the support for FUA.
> >
> > I'm not sure it's worth it. The pure REQ_FLUSH path works not and is
> > well tested with kvm/qemu. We can still easily add a FUA bit, and
> > even a pre-flush bit if the protocol roundtrips matter in real life
> > benchmarking.
>
> Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
> should be cheaper than FLUSH.

If someone ever wrote a virtio-blk backend that sits directly ontop
of the Linux block layer that would be true. Of the five known
virtio-blk backends all operate on normal files using the Posix I/O
APIs, or the Linux aio API (optionally in qemu) or in-kernel
vfs_read/vfs_write (vhost-blk).

Given how little testing lguest gets compared to qemu I really don't
want a protocol addition for it unless it really buys us something.
Once we're done with this barrier conversion I plan into benchmarking
FUA and a pre-flush tag on the command for virtio in real life setups,
and see if it actually buys us anything.

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Old 08-17-2010, 04:22 PM
Tejun Heo
 
Default virtio_blk: implement REQ_FLUSH/FUA support

Hello,

On 08/17/2010 03:23 PM, Christoph Hellwig wrote:
>> Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
>> should be cheaper than FLUSH.
>
> If someone ever wrote a virtio-blk backend that sits directly ontop
> of the Linux block layer that would be true. Of the five known
> virtio-blk backends all operate on normal files using the Posix I/O
> APIs, or the Linux aio API (optionally in qemu) or in-kernel
> vfs_read/vfs_write (vhost-blk).

Right.

> Given how little testing lguest gets compared to qemu I really don't
> want a protocol addition for it unless it really buys us something.
> Once we're done with this barrier conversion I plan into benchmarking
> FUA and a pre-flush tag on the command for virtio in real life setups,
> and see if it actually buys us anything.

Hmmm... yeah, we can drop it. Michael, what do you think?

Thanks.

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Old 08-18-2010, 10:22 AM
Rusty Russell
 
Default virtio_blk: implement REQ_FLUSH/FUA support

On Tue, 17 Aug 2010 10:53:27 pm Christoph Hellwig wrote:
> Given how little testing lguest gets compared to qemu I really don't
> want a protocol addition for it unless it really buys us something.
> Once we're done with this barrier conversion I plan into benchmarking
> FUA and a pre-flush tag on the command for virtio in real life setups,
> and see if it actually buys us anything.

Absolutely. Lguest should follow, not lead!

Rusty.

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