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 virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
