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Re: [Qemu-block] [Qemu-devel] Persistent bitmaps for non-qcow2 formats


From: Yaniv Lavi (Dary)
Subject: Re: [Qemu-block] [Qemu-devel] Persistent bitmaps for non-qcow2 formats
Date: Tue, 29 Aug 2017 12:26:06 +0300



YANIV LAVI (YANIV DARY)

SENIOR TECHNICAL PRODUCT MANAGER

Red Hat Israel Ltd.

34 Jerusalem Road, Building A, 1st floor

Ra'anana, Israel 4350109

address@hidden    T: +972-9-7692306/8272306     F: +972-9-7692223    IM: ylavi

TRIED. TESTED. TRUSTED.

On Mon, Aug 28, 2017 at 9:11 PM, John Snow <address@hidden> wrote:


On 08/27/2017 10:57 PM, Fam Zheng wrote:
> On Fri, 08/25 15:44, Max Reitz wrote:
>> Well, OK.  The main argument against supporting anything but qcow2 is
>> "if you want features, use qcow2; and we are working on making qcow2 as
>> fast as possible."  I think that's a very good argument still.  At some
>> point I (and probably others, too) had the idea of making qcow2 files in
>> raw layout:
>
>
> Yes! I think this idea makes a whole lot of sense, too. Metadata tables can be
> generated so old implementation can still use it.
>
> Fam
>
>> Have the data as a blob, just like a raw file, padded by
>> metadata around it.  An autoclear flag would specify that the qcow2 file
>> is in this format, and if so, you could simply access it like a raw file
>> and should have exactly the same speed as a raw file.  Maybe that would
>> solve this whole issue, too?

I wonder if this would be sufficient to alleviate the desire to use raw
files...

(Eh, well, realistically, someone's still always going to ask if they
can use various features with non-qcow2 files...)

Nir, Yaniv; any input?

We are using raw format for performance reasons.
As we have many customers that currently use this format, not support it would be a blocker the use of the feature.
At a minimum we would require ability to convert raw to qcow2 raw-layout.

Please also consider that we are planning to go on the OSP route of LUN per disk and would still want the tracking to work.
I makes sense that for that and raw format you will be able to save the mapping to another file other than a qcow.
 

(Context: We're debating how to add persistent bitmaps to raw files as I
was informed that RHV was 'asking about it.' Max is reminding me there
is a proposal for a style of QCOW2 that uses a raw layout for data,
mitigating or eliminating any performance hits related to the L2 cache.
What I am not aware of is why RHV would use raw files for any purpose.
Is it performance? Simplicity? Could RHV use a raw-layout qcow2?)

--js


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