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Re: [Nbd] BCache with NBD is it possible?



On Tue, Feb 25, 2014 at 09:30:09AM +0100, Juan Antonio Martinez wrote:
> El lun, 24-02-2014 a las 09:59 -0500, Paul Clements escribió:
> > It certainly should work. bcache is a generic block device cache. That
> > said, I have not actually tried it with nbd, so maybe there's some bug
> > preventing it from working?
> > 
> Well, AFAIK the main problem is that nbd real device is a file with an
> in-built squashfs that resides in a remote server... 
> 
> That is: I can create my local cache device by mean of "make-bcache
> -C /dev/sda1" but I cannot declare caching device by mean of
> "make-bcache -B /dev/nbd0" because returns "device busy" error.... thus
> cannot attach it to my local cache device . 
> 
> ¿Any way to bypass this? Perhaps I'm missing something about how bcache
> works... :-(
> 
> Thanks for your attention. Cheers
> Juan Antonio
> 
> > On Mon, Feb 24, 2014 at 7:46 AM, Juan Antonio Martinez
> > <jantonio@...1345...> wrote:
> >         Hi all:
> >         I have 250+ ubuntu 13.10 ltsp fat clients. As client hardware
> >         is a "bit
> >         obsolete" and don't want to buy extra memory for those
> >         computers, we are
> >         studying some configurations to improve performance.
> >         
> >         As on 13.10 BCache comes in the kernel mainline we'll want to
> >         use local
> >         client's disk as cache for nbd device.
> >         
> >         Is it possible? BCache documentation suggest it, but didn't
> >         find any
> >         tutorial/guide. Everything I got is a message like
> >         "Device /dev/nbd0 is
> >         not a partition, a logical volume or a LUKS volume" when using
> >         "blocks
> >         to-cache" commannd
> >         
> >         Thanks in advance
> >         Juan Antonio

"device busy" error sounds like you have the nbd0 already mounted. You
need to setup bcache before mounting it, maybe from an initramfs.

"Device /dev/nbd0 is not a partition, a logical volume or a LUKS
volume" would indicate that the tool expects device names to match a
certain pattern, like sd[a-z][1-9], and does not know about nbd0.
Which is a bit stupid to do since the name is totaly arbitrary. Check
the source for where and why it generates that error and fix that.

MfG
	Goswin



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