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Re: [Xen-API] RE: Questions about the api

To: Dave Scott <Dave.Scott@xxxxxxxxxxxxx>
Subject: Re: [Xen-API] RE: Questions about the api
From: Julian Chesterfield <julian.chesterfield@xxxxxxxxxxxxx>
Date: Thu, 21 Oct 2010 18:29:37 +0100
Cc: 'Tomoiaga Cristian' <cristi@xxxxxxxxxxx>, "xen-api@xxxxxxxxxxxxxxxxxxx" <xen-api@xxxxxxxxxxxxxxxxxxx>
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On 21/10/10 18:16, Dave Scott wrote:
Hi,

Hello everyone,

I am writing a SR for Nexenta since I haven't found one and zfs seems
pretty good until now.
My questions are more related to missing(?) api calls.
I need to know if I can set the following fields in a VDI for example:

Is-a-snapshot - there is a get_is_a_snapshot call but not a
set_is_a_snapshot. Same for snapshot-of. Think it's usefull just in
case I vdi-forget the snapshot and re introduce it with a scan or in
other situations.
parent - there is a get_parent but not a set_parent call, this would be
nice.
Hm, I think there's no current way for the backend to set these VDI fields: 
they seem to be only ever set by xapi at creation time. You're right that if 
you forget and then re-scan then the metadata will be lost. For most backends 
this isn't a fatal problem (although it might be annoying) but I wonder what 
will happen for LVHD -- it uses the 'is_a_snapshot' as an indicator to decide 
whether to 'inflate' or 'deflate' the LVM LV.
That's true. Of course the issue is less critical (but still annoying) if LVHD provides thin provisioning. I've thought for a while that it would be beneficial to store more metadata persistently within the SR itself, to make it more portable and to overcome synchronisation issues like this. This was the original intention for the MGT volume in LVHD.

We should consider allowing the backend to read/write these fields. There's 
already precedent for that kind of thing: the backend can call 
VDI.set_virtual_size but it looks read/only to everyone else.
I would be strongly in favour of this also.

- Julian
What do others think?

Dave


Of course I can use sm-config for example, to set a parent but still,
since the field is there I would like to set it myself.

Thank you!
Tomoiaga Cristian


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