On Tue, Sep 04, 2018 at 07:55:00PM -0600, Chris Murphy wrote:
> https://rwmj.wordpress.com/2018/09/04/nbdkit-as-a-flexible-alternative-to-loopback-mounts/
>
> This is a pretty cool writeup. I can vouch Btrfs will format mount,
> write to, scrub, and btrfs check works on an 8EiB (virtual) disk.
>
> The one thing I thought might cause a problem is the ndb device has a
> 1KiB sector size, but Btrfs (on x86_64) still uses 4096 byte "sector"
> and it all seems to work fine despite that.
Thanks for the kind words. I did an updated post verifying what you
said and also noting that the ‘nbd-client -b’ option can be used to
adjust the sector size:
https://rwmj.wordpress.com/2018/09/05/nbdkit-for-loopback-pt-5-8-exabyte-btrfs-filesystem/
Btrfs still seems to believe the sector size is 4k, although as you
say it doesn't seem to cause any issues.
> Anyway, maybe it's useful for some fstests instead of file backed
> losetup devices?
One interesting feature of nbdkit is that you can write your own
plugins. For my demonstration, I used the nbdkit-memory-plugin which
implements a purely in-memory sparse array:
https://github.com/libguestfs/nbdkit/blob/master/plugins/memory/memory.c
https://github.com/libguestfs/nbdkit/tree/master/common/sparse
But to test btrfs you might want to write a custom plugin. For
example you might choose a sparse array implementation which is more
suitable for storing specifically btrfs metadata structures, or can
spill to a disk file (which nbdkit-memory-plugin cannot, except swap).
Another thing that's interesting from a testing point of view is the
ability to inject block device errors on demand. You can either do
this using the supplied nbdkit-error-filter:
https://rwmj.wordpress.com/2018/09/04/nbdkit-for-loopback-pt-2-injecting-errors/
or if you were writing your own plugin you'd probably want to do it
there.
Anyway hope you find it interesting.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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