Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Blaisorblade]

On Wednesday 21 September 2005 17:48, Alasdair G Kergon wrote:
> On Tue, Sep 20, 2005 at 04:34:33PM -0700, Andrew Morton wrote:
> > ack?
>
> Applied some quick corrections to this.
I'm now looking at this, but you've done an error. You say that 

You must create the snapshot-origin device before you can create snapshots.

This is totally wrong. The whole point of my document is to show that you can 
create snapshots *WITHOUT* having a snapshot-origin device (yes, I did it).

And while the LVM setup (with snapshot-origin) is often used and well known, 
the possibility to do without the snapshot-origin, and more important to 
write on the snapshots, is something which is not widely known at all.

In fact, UML community has been using an in-house version of the same feature, 
which cannot be mounted on the host due to missing code.
> See also http://people.redhat.com/agk/talks/FOSDEM_2005/
> (slides 15-27)
Not yet had time to look at those, sorry.
> You might like to convert some of those diagrams to ASCII
> and include them?
If you feel like doing this, ok, but I'm not normally a documentor.
> Alasdair

-- 
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Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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[uml-devel] Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Blaisorblade]

On Wednesday 21 September 2005 17:48, Alasdair G Kergon wrote:
> On Tue, Sep 20, 2005 at 04:34:33PM -0700, Andrew Morton wrote:
> > ack?
>
> Applied some quick corrections to this.
I'm now looking at this, but you've done an error. You say that 

You must create the snapshot-origin device before you can create snapshots.

This is totally wrong. The whole point of my document is to show that you can 
create snapshots *WITHOUT* having a snapshot-origin device (yes, I did it).

And while the LVM setup (with snapshot-origin) is often used and well known, 
the possibility to do without the snapshot-origin, and more important to 
write on the snapshots, is something which is not widely known at all.

In fact, UML community has been using an in-house version of the same feature, 
which cannot be mounted on the host due to missing code.
> See also http://people.redhat.com/agk/talks/FOSDEM_2005/
> (slides 15-27)
Not yet had time to look at those, sorry.
> You might like to convert some of those diagrams to ASCII
> and include them?
If you feel like doing this, ok, but I'm not normally a documentor.
> Alasdair

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Alasdair G Kergon]

On Fri, Sep 23, 2005 at 10:11:31PM +0200, Blaisorblade wrote:
> you can create snapshots *WITHOUT* having a snapshot-origin device 
 
Checking the code, yes you're correct.
When that LVM2 code was developed you'd get an oops but it's been fixed.

Alasdair
-- 
agk@redhat.com

[uml-devel] Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Alasdair G Kergon]

On Fri, Sep 23, 2005 at 10:11:31PM +0200, Blaisorblade wrote:
> you can create snapshots *WITHOUT* having a snapshot-origin device 
 
Checking the code, yes you're correct.
When that LVM2 code was developed you'd get an oops but it's been fixed.

Alasdair
-- 
agk@redhat.com


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Re: [uml-devel] Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Blaisorblade]

On Friday 23 September 2005 23:19, Alasdair G Kergon wrote:
> On Fri, Sep 23, 2005 at 10:11:31PM +0200, Blaisorblade wrote:
> > you can create snapshots *WITHOUT* having a snapshot-origin device

> Checking the code, yes you're correct.
> When that LVM2 code was developed you'd get an oops but it's been fixed.
You also "fixed" my assertion that snapshot-origin "base device" (parameter 
n.1) must be a DM device. Should I readd it too?

IIRC I checked the code and it looks up the first param as a DM device, even 
because the list of snapshot is maintained for DM devices, not for general 
block devices.

Right?
> Alasdair

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade


		
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Re: [uml-devel] Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Blaisorblade]

On Friday 23 September 2005 23:19, Alasdair G Kergon wrote:
> On Fri, Sep 23, 2005 at 10:11:31PM +0200, Blaisorblade wrote:
> > you can create snapshots *WITHOUT* having a snapshot-origin device

> Checking the code, yes you're correct.
> When that LVM2 code was developed you'd get an oops but it's been fixed.
You also "fixed" my assertion that snapshot-origin "base device" (parameter 
n.1) must be a DM device. Should I readd it too?

IIRC I checked the code and it looks up the first param as a DM device, even 
because the list of snapshot is maintained for DM devices, not for general 
block devices.

Right?
> Alasdair

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade


		
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Re: [uml-devel] Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Alasdair G Kergon]

On Mon, Sep 26, 2005 at 05:03:01PM +0200, Blaisorblade wrote:
> You also "fixed" my assertion that snapshot-origin "base device" (parameter 
> n.1) must be a DM device. Should I readd it too?
 
That parameter does not need to be a DM device if you are able to use
the start of an existing block device (so you don't require any mappings).

> IIRC I checked the code and it looks up the first param as a DM device, 

It should only use the device number for reference, not any dm structure.

Try it.

Alasdair
-- 
agk@redhat.com

Re: [uml-devel] Re: Writing on DM snapshots, and having no "mainstream" device (was: Re: Fw: [PATCH 1/7] Add dm-snapshot tutorial in Documentation)

[Alasdair G Kergon]

On Mon, Sep 26, 2005 at 05:03:01PM +0200, Blaisorblade wrote:
> You also "fixed" my assertion that snapshot-origin "base device" (parameter 
> n.1) must be a DM device. Should I readd it too?
 
That parameter does not need to be a DM device if you are able to use
the start of an existing block device (so you don't require any mappings).

> IIRC I checked the code and it looks up the first param as a DM device, 

It should only use the device number for reference, not any dm structure.

Try it.

Alasdair
-- 
agk@redhat.com


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