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

[Gabriel Niebler <gniebler@suse.com>]

Am 18.02.23 um 07:26 schrieb Zorro Lang:
> 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.

Fine by me. Users will figure it out when running the tests and reading 
the skip reasons.