Re: btrfs seed question

[Chris Murphy <lists@colorremedies.com>]

On Thu, Oct 12, 2017 at 2:20 PM, Joseph Dunn <jdunn14@gmail.com> wrote:
>
> On Thu, 12 Oct 2017 12:18:01 +0800
> Anand Jain <anand.jain@oracle.com> wrote:
>
> > On 10/12/2017 08:47 AM, Joseph Dunn wrote:
> > > After seeing how btrfs seeds work I wondered if it was possible to push
> > > specific files from the seed to the rw device.  I know that removing
> > > the seed device will flush all the contents over to the rw device, but
> > > what about flushing individual files on demand?
> > >
> > > I found that opening a file, reading the contents, seeking back to 0,
> > > and writing out the contents does what I want, but I was hoping for a
> > > bit less of a hack.
> > >
> > > Is there maybe an ioctl or something else that might trigger a similar
> > > action?
> >
> >    You mean to say - seed-device delete to trigger copy of only the
> > specified or the modified files only, instead of whole of seed-device ?
> > What's the use case around this ?
> >
>
> Not quite.  While the seed device is still connected I would like to
> force some files over to the rw device.  The use case is basically a
> much slower link to a seed device holding significantly more data than
> we currently need.  An example would be a slower iscsi link to the seed
> device and a local rw ssd.  I would like fast access to a certain subset
> of files, likely larger than the memory cache will accommodate.  If at
> a later time I want to discard the image as a whole I could unmount the
> file system or if I want a full local copy I could delete the
> seed-device to sync the fs.  In the mean time I would have access to
> all the files, with some slower (iscsi) and some faster (ssd) and the
> ability to pick which ones are in the faster group at the cost of one
> content transfer.


Multiple seeds?

Seed A has everything, is remote. Create sprout B also remotely,
deleting the things you don't absolutely need, then make it a seed.
Now via iSCSI you can mount both A and B seeds. Add local rw sprout C
to seed B, then delete B to move files to fast local storage.



-- 
Chris Murphy