Re: Disk naming (Was Re: [Xen-devel] [PATCH] Guest boot loadersupport [1/2])

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > OK, it's not a very cunning hack to do but it makes the blkback driver
> > more complex whilst duplicating functionality that's already in the
> > kernel.  You'd have to deal with a load of VFS APIs as well as the
> > existing block APIs, which would be unfortunate.
>
> Maybe reuse the loopback code?  Maybe it has an abstract device->file
> conversion group of functions/structures.

Arguably the ideal in-kernel solution is to fix the loopback driver itself.

> > Does anybody know why the existing loopback device performs badly anyway?
> >  As Adam says, it shouldn't be rocket science to make it work well...
>
> Default kernel limit of 8 loops.  Requires a recompile, and then probably
> only supports 256 devices max.

Yup the default limit could (arguably) be raised in our default config.  I was 
especially referring to the performance, which is reported to be rather bad.

> > > blktap?  Is that similiar to what I described above?
> >
> > Blktap allows you to write a userspace program to provide a block device
> > to another domain.  It makes what you described a bit more
> > straightforward than implementing directly in the blkback driver.
>
> That would be more complex I would think then doing file access in the
> kernel.

I'm not sure it would, the blocktap library is designed for this sort of 
thing.  From what Andy says there is actually already a tool to do it, which 
might be worth investigating further.

Cheers,
Mark

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