blkback name greater than 16 characters issue

blkback name greater than 16 characters issue

[Konrad Rzeszutek Wilk]

Hey,

Way back in the past Jan pointed out this issue
http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03505.html

which is that the device name is "blkback.<domid>.<name>"

for exmaple: blkback.8.xvda, blkback.11.hda

With the multiqueue block backend we add "-%d" for the queue. 
But sadly this is already way past the limit so it gets stripped.


Possible solution had been identified by Ian:
http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03516.html

	"
	If you are pressed for space then the "xvd" is probably a bit redundant
	in a string which starts blkbk.

	The guest may not even call the device xvdN (iirc BSD has another
	prefix) any how, so having blkback say so seems of limited use anyway.

	Since this seems to not include a partition number how does this work in
	the split partition scheme? (i.e. one where the guest is given xvda1 and
	xvda2 rather than xvda with a partition table)

[It will be 'blkback.8.xvda1', and 'blkback.11.xvda2']

	Perhaps something derived from one of the schemes in
	http://xenbits.xen.org/docs/unstable/misc/vbd-interface.txt might be a
	better fit?
"

[Inline is my answer]

My desire is to easily correlate the name of the thread with the
guest config. And that means preserving the 'xvda' or 'hda' or whatever
the user had in mind.

I believe the domain id is important as well - so we need that.

Which means we are left with the prefix. Should it be 'blkbk' or
should it be completely eliminated? Meaning we have:

 8.xvda
 11.hda

 and
 9.xvda-1
 9.xvda-2
 32100.xvdfg9-5

Or perhaps use 'blk."

blk.32100.xvdfg9-5
(exactly 17, so '5' is gone).

Perhaps in those occassions drop 'blk.' altogether? But I wouldn't want
a special case, so back to:

 32100.xvdfg9-5

? Thoughts?

Or 

Re: blkback name greater than 16 characters issue

[Wei Liu]

On Mon, Dec 14, 2015 at 11:46:35AM -0500, Konrad Rzeszutek Wilk wrote:
> Hey,
> 
> Way back in the past Jan pointed out this issue
> http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03505.html
> 
> which is that the device name is "blkback.<domid>.<name>"
> 
> for exmaple: blkback.8.xvda, blkback.11.hda
> 
> With the multiqueue block backend we add "-%d" for the queue. 
> But sadly this is already way past the limit so it gets stripped.
> 
> 
> Possible solution had been identified by Ian:
> http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03516.html
> 
> 	"
> 	If you are pressed for space then the "xvd" is probably a bit redundant
> 	in a string which starts blkbk.
> 
> 	The guest may not even call the device xvdN (iirc BSD has another
> 	prefix) any how, so having blkback say so seems of limited use anyway.
> 
> 	Since this seems to not include a partition number how does this work in
> 	the split partition scheme? (i.e. one where the guest is given xvda1 and
> 	xvda2 rather than xvda with a partition table)
> 
> [It will be 'blkback.8.xvda1', and 'blkback.11.xvda2']
> 
> 	Perhaps something derived from one of the schemes in
> 	http://xenbits.xen.org/docs/unstable/misc/vbd-interface.txt might be a
> 	better fit?
> "
> 
> [Inline is my answer]
> 
> My desire is to easily correlate the name of the thread with the
> guest config. And that means preserving the 'xvda' or 'hda' or whatever
> the user had in mind.
> 
> I believe the domain id is important as well - so we need that.
> 
> Which means we are left with the prefix. Should it be 'blkbk' or
> should it be completely eliminated? Meaning we have:
> 
>  8.xvda
>  11.hda
> 
>  and
>  9.xvda-1
>  9.xvda-2
>  32100.xvdfg9-5
> 
> Or perhaps use 'blk."
> 
> blk.32100.xvdfg9-5
> (exactly 17, so '5' is gone).
> 
> Perhaps in those occassions drop 'blk.' altogether? But I wouldn't want
> a special case, so back to:
> 
>  32100.xvdfg9-5
> 
> ? Thoughts?
> 
> Or 
> 

? Or enlarge TASK_COMM_LEN? :-)

Jokes aside, 'git grep blkback' in tools directory doesn't show any
reference to "blkback" in code (as I would expect), so there is no risk
in breaking tookstack should you change the name.

I don't really have a preference whatsoever on this.

Wei.

Re: blkback name greater than 16 characters issue

[Ian Jackson]

Konrad Rzeszutek Wilk writes ("blkback name greater than 16 characters issue"):
> Way back in the past Jan pointed out this issue
> http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03505.html
> 
> which is that the device name is "blkback.<domid>.<name>"
> for exmaple: blkback.8.xvda, blkback.11.hda

I don't understand in what sense this is the `device name'.

AFAICT from your references we are talking about what appears in `ps'
on BSD.  Is that right ?  The lack of uniqueness is a problem for
admins and for debugging but not for functionality ?

> My desire is to easily correlate the name of the thread with the
> guest config. And that means preserving the 'xvda' or 'hda' or whatever
> the user had in mind.
> 
> I believe the domain id is important as well - so we need that.
> 
> Which means we are left with the prefix. Should it be 'blkbk' or
> should it be completely eliminated? Meaning we have:

Surely this string should have something in it which makes it clear
that it's a Xen vbd backend.

I suggest for xvds

  1234567890123456
  DOMID.xvdXYNN

and for the others

  1234567890123456
  DOMID.xvd-hdXYNN

It's a shame that DOMID comes first but I can't think of a sane syntax
that puts `x', DOMID, type, and XYNN in that order.

Ian.

Re: blkback name greater than 16 characters issue

[Konrad Rzeszutek Wilk]

On Mon, Jan 04, 2016 at 04:13:57PM +0000, Ian Jackson wrote:
> Konrad Rzeszutek Wilk writes ("blkback name greater than 16 characters issue"):
> > Way back in the past Jan pointed out this issue
> > http://lists.xenproject.org/archives/html/xen-devel/2015-05/msg03505.html
> > 
> > which is that the device name is "blkback.<domid>.<name>"
> > for exmaple: blkback.8.xvda, blkback.11.hda
> 
> I don't understand in what sense this is the `device name'.
> 
> AFAICT from your references we are talking about what appears in `ps'
> on BSD.  Is that right ?  The lack of uniqueness is a problem for
> admins and for debugging but not for functionality ?

Correct. Just for troubleshooting or such. You can look at 'top'
or 'ps' and see which of the blkback threads is eating CPU cycles or such.

> 
> > My desire is to easily correlate the name of the thread with the
> > guest config. And that means preserving the 'xvda' or 'hda' or whatever
> > the user had in mind.
> > 
> > I believe the domain id is important as well - so we need that.
> > 
> > Which means we are left with the prefix. Should it be 'blkbk' or
> > should it be completely eliminated? Meaning we have:
> 
> Surely this string should have something in it which makes it clear
> that it's a Xen vbd backend.
> 
> I suggest for xvds
> 
>   1234567890123456
>   DOMID.xvdXYNN
> 
> and for the others
> 
>   1234567890123456
>   DOMID.xvd-hdXYNN
> 
> It's a shame that DOMID comes first but I can't think of a sane syntax
> that puts `x', DOMID, type, and XYNN in that order.

OK, let me prep a patch for this shortly.

Thank you!
> 
> Ian.