Re: [PATCH 2/2] README: Add tiny note concerning required scratch devs for btrfs testing

[Zorro Lang <zlang@redhat.com>]

On Mon, Feb 06, 2023 at 05:29:14PM +0100, Gabriel Niebler wrote:
> Am 06.02.23 um 15:52 schrieb Zorro Lang:
> > On Tue, Jan 31, 2023 at 01:38:39PM +0100, Gabriel Niebler wrote:
> > > Signed-off-by: Gabriel Niebler <gniebler@suse.com>
> > > ---
> > >   README | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/README b/README
> > > index 008d3875..574cdd34 100644
> > > --- a/README
> > > +++ b/README
> > > @@ -127,7 +127,7 @@ Setup Environment
> > >       - device contents will be destroyed.
> > >   4. (optional) Create SCRATCH device pool.
> > > -    - needed for BTRFS testing
> > > +    - needed for BTRFS testing (some tests require up to 5 disks of 10GB each)
> > 
> > Not sure about this part, so btrfs recommend 5 disks of 10GB each in
> > SCRATCH_DEV_POOL, to run all btrfs related tests? Does anyone know that?
> > Or can you provide the subcase names which need "5 disks of 10GB each",
> > to help to check and review?
> 
> Of course (and thanks for asking, as it turns out my recollection was
> slightly wrong on that one):
> 
> To begin with, btrfs/011[1] states the "5 disks" requirement in the lengthy
> comment at the top of the file:
> """
> # The amount of tests done depends on the number of devices in the
> # SCRATCH_DEV_POOL. For full test coverage, at least 5 devices should
> # be available (e.g. 5 partitions).
> ""
> ... but it doesn't say anything about device size.
> 
> btrfs/027[2] however checks the presence of 5 devices in $SCRATCH_DEV_POOL
> directly in code and also runs a check to ensure that all of these devices
> are *of the same size*.
> 
> And then there are at least two tests that require $SCRATCH_DEV to be 10GB
> or more in size:
> - btrfs/079[3], which writes a 10G file to $SCRATCH_MNT and
> - btrfs/253[4], which directly creates a 10G FS on $SCRATCH_DEV.
> 
> *Taken together*, this means that for full btrfs test coverage, one needs a
> $SCRATCH_DEV_POOL with at least 5 devices of equal size in it and that size
> has to be at least 10GB.
> 
> But that makes the remark wrong, strictly speaking, as there's no single
> test that requires "at least 5 devices of 10GB each".
> 
> How about this: "(for full btrfs test coverage, use at least 5 devices of
> equal size and at least 10GB)"

Even for now 10GB is enough, but we can't always assue that. There's not a clear
restriction about that. I think we can skip this sub-patch. At least we don't
give others a recommendation which isn't assured by ourselves. So how about only
merge the 1st patch.

> 
> > P.S. I'm thinking about if this "(...)" should be moved to below item.
> >                                                          vvvvv
> > >       - specifies 3 or more independent SCRATCH devices via the SCRATCH_DEV_POOL
> > >         variable e.g SCRATCH_DEV_POOL="/dev/sda /dev/sdb /dev/sdc"
> 
> Fine by me! :-)
> 
> [1]: https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/tests/btrfs/011#n10
> [2]: https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/tests/btrfs/027#n20
> [3]: https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/tests/btrfs/079#n44
> [4]: https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/tree/tests/btrfs/253#n90
>