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Re: [Qemu-devel] [RFC] Disk integrity in QEMU
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [RFC] Disk integrity in QEMU |
Date: |
Tue, 14 Oct 2008 17:21:40 +0200 |
User-agent: |
Thunderbird 2.0.0.16 (X11/20080723) |
Anthony Liguori wrote:
> Avi Kivity wrote:
>> Given that we don't have a zero-copy implementation yet, it is
>> impossible to generate real performance data.
>
> Which means that it's premature to suggest switching the default to
> O_DIRECT since it's not going to help right now. It can be revisited
> once we can do zero copy but again, I think it should be driven by
> actual performance data. My main point is that switching to O_DIRECT
> right now is only going to hurt performance for some users, and most
> likely help no one.
I am assuming that we will provide true O_DIRECT support soon.
I don't think O_DIRECT should be qemu's default, since anyone using qemu
directly is likely a "causal virtualization" user. Management systems
like ovirt should definitely default to O_DIRECT (really, they shouldn't
even offer caching).
>> Now I don't have data that demonstrates how bad these effects are, but I
>> think there is sufficient arguments here to justify adding O_DIRECT. I
>> intend to recommend O_DIRECT unless I see performance data that favours
>> O_DSYNC on real world scenarios that take into account bandwidth, cpu
>> utilization, and memory utilization (i.e. a 1G guest on a 32G host
>> running fio but not top doesn't count).
>>
>
> So you intend on recommending something that you don't think is going
> to improve performance today and you know in certain scenarios is
> going to decrease performance? That doesn't seem right :-)
>
In the near term O_DIRECT will increase performance over the alternative.
> I'm certainly open to changing the default once we get to a point
> where there's a demonstrable performance improvement from O_DIRECT but
> since I don't think it's a given that there will be, switching now
> seems like a premature optimization which has the side effect of
> reducing the performance of certain users. That seems like a Bad
> Thing to me.
I take the opposite view. O_DIRECT is the, well, direct path to the
hardware. Caching introduces an additional layer of code and thus needs
to proven effective. I/O and memory intensive applications use
O_DIRECT; Xen uses O_DIRECT (or equivalent); I don't see why we need to
deviate from industry practice.
--
Do not meddle in the internals of kernels, for they are subtle and quick to
panic.
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, (continued)
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Andrea Arcangeli, 2008/10/15
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU,
Avi Kivity <=
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Anthony Liguori, 2008/10/14
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/14
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Laurent Vivier, 2008/10/14
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/16
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Izik Eidus, 2008/10/13
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/14
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/12
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Jens Axboe, 2008/10/17
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Avi Kivity, 2008/10/19
- Re: [Qemu-devel] [RFC] Disk integrity in QEMU, Jens Axboe, 2008/10/19