Re: Formatting of backing device

[Piergiorgio Sartor <piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org>]

Hi Adam,

On Wed, Feb 01, 2012 at 01:38:12PM -0800, Adam Berkan wrote:
> Oh, sorry I misunderstood.
> 
> You have to run make-bcache once to add a bcache superblock to the drive.
>  After that the drive contents are destroyed and it needs to be formatted
> with a filesystem.

ah! That's not good...

Is there any plan to have the caching device attachable
and detachable from *any* backing device without prior
"formatting" of this second one?

I think bcache is a very interesting and promising
project, but formatting the backing device is
something, I think, that should be avoided.

bye,

pg

> At that point you can attach or detach the drive while it is in use.
> 
> Adam
> 
> On Wed, Feb 1, 2012 at 12:54 PM, Piergiorgio Sartor <
> piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
> 
> > Hi Adam,
> >
> > thanks for the answer, see below.
> >
> > On Wed, Feb 01, 2012 at 11:04:59AM -0800, Adam Berkan wrote:
> > > You can attach bcache to a drive with an existing file system, and it
> > will
> > > continue as normal.  If you connect to a drive without a file system,
> > then
> > > it will continue to not have a file system, but you can format it while
> > > attached.
> >
> > Maybe I misused the term "format".
> >
> > I did not mean filesystem format, but bcache format.
> >
> > What I understood, maybe I'm wrong, is that the backing
> > device, before being used, must be "initialized" with
> > the bcache tool.
> >
> > From the docs:
> >
> > Getting started:
> > You'll need make-bcache from the bcache-tools repository. Both the cache
> > device
> > and backing device must be formatted before use.
> >  make-bcache -B /dev/sdb
> >  make-bcache -C -w2k -b1M -j64 /dev/sdc
> >
> > I understand this as the backing device gets something
> > on written on it (note the term "formatted").
> >
> > Am I wrong? I hope so...
> >
> > Thanks again,
> >
> > bye,
> >
> > pg
> >
> > > Attach/detach should work while the device is in use.  This isn't the
> > most
> > > tested code path, especially with writeback on, but it's supposed to
> > work.
> > >  Detaching while the cache is dirty requires flushing all that data so
> > > performance will be bad until the detach completes.
> > >
> > > Let us know if you find any bugs.
> > > Adam
> > >
> > > On Wed, Feb 1, 2012 at 2:10 AM, Piergiorgio Sartor <
> > > piergiorgio.sartor-KvP5wT2u2U0@public.gmane.org> wrote:
> > >
> > > > Hi all,
> > > >
> > > > first of all I would like to congratulate for this
> > > > project, I think it is one of the most promising
> > > > feature the Linux kernel can have.
> > > >
> > > > Wrote that, I've a question about the concept of
> > > > formatting the backing device.
> > > >
> > > > As far as I understood, the first concept of bcache
> > > > was to simply "register" or "attach" a cache to a
> > > > backing device, that is, the backing device had not
> > > > to be formatted.
> > > >
> > > > Lately, still if I understood it correctly, this
> > > > behaviour was changed and, now, the backing device
> > > > needs to be formatted.
> > > >
> > > > So, the question is:
> > > >
> > > > How about an already running device? Is it still
> > > > possible to attach a cache under such situation?
> > > >
> > > > In general, would it be possible to attach/detach
> > > > a cache to any already available device (in the
> > > > future)? Or the caching/backing setup must be planned
> > > > before the HW is available, so to speak?
> > > >
> > > > It would be useful (and cool too), to have the
> > > > possibility to attach/detach the SSD cache, on
> > > > the fly (at run-time) to any device it needs it.
> > > >
> > > > I hope the question(s) are clear, if not please
> > > > let me know.
> > > >
> > > > Thanks a lot in advance,
> > > >
> > > > bye,
> > > >
> > > > --
> > > >
> > > > piergiorgio
> > > > --
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> >
> > --
> >
> > piergiorgio
> >

-- 

piergiorgio