Re: [PATCH] btrfs: allow degenerate raid0/raid10

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2021/7/23 下午10:08, David Sterba wrote:
> On Fri, Jul 23, 2021 at 06:51:31AM +0800, Qu Wenruo wrote:
>>
>>
>> On 2021/7/23 上午3:29, David Sterba wrote:
>>> The data on raid0 and raid10 are supposed to be spread over multiple
>>> devices, so the minimum constraints are set to 2 and 4 respectively.
>>> This is an artificial limit and there's some interest to remove it.
>>
>> This could be a better way to solve the SINGLE chunk created by degraded
>> mount.
>
> Yes, but in this case it's rather a conicidence because raid0 now
> becomes a valid fallback profile, other cases are not affected. There's
> also some interest to allow full write with missing devices (as long as
> complete data can be written, not necessarily to all copies). MD-RAID
> allows that.
>
> As an example, when we'd allow that, 2 device raid1 with one missing
> will continue to write to the present device and once the missing device
> reappears, scrub will fill the missing bits, or device replace will do a
> full sync.
>
>>> Change this to allow raid0 on one device and raid10 on two devices. This
>>> works as expected eg. when converting or removing devices.
>>>
>>> The only difference is when raid0 on two devices gets one device
>>> removed. Unpatched would silently create a single profile, while newly
>>> it would be raid0.
>>>
>>> The motivation is to allow to preserve the profile type as long as it
>>> possible for some intermediate state (device removal, conversion).
>>>
>>> Unpatched kernel will mount and use the degenerate profiles just fine
>>> but won't allow any operation that would not satisfy the stricter device
>>> number constraints, eg. not allowing to go from 3 to 2 devices for
>>> raid10 or various profile conversions.
>>
>> My initial thought is, tree-checker will report errors like crazy, but
>> no, the check for RAID1 only cares substripe, while for RAID0 no number
>> of devices check.
>>
>> So a good surprise here.
>>
>> Another thing is about the single device RAID0 or 2 devices RAID10 is
>> the stripe splitting.
>>
>> Single device RAID0 is just SINGLE, while 2 devices RAID10 is just RAID1.
>> Thus they need no stripe splitting at all.
>>
>> But we will still do the stripe calculation, thus it could slightly
>> reduce the performance.
>> Not a big deal though.
>
> Yeah effectively they're raid0 == single, raid10 == raid1, I haven't
> checked the overhead of the additional striping logic nor measured
> performance impact but I don't feel it would be noticeable.
>
>>> Example output:
>>>
>>>     # btrfs fi us -T .
>>>     Overall:
>>>         Device size:                  10.00GiB
>>>         Device allocated:              1.01GiB
>>>         Device unallocated:            8.99GiB
>>>         Device missing:                  0.00B
>>>         Used:                        200.61MiB
>>>         Free (estimated):              9.79GiB      (min: 9.79GiB)
>>>         Free (statfs, df):             9.79GiB
>>>         Data ratio:                       1.00
>>>         Metadata ratio:                   1.00
>>>         Global reserve:                3.25MiB      (used: 0.00B)
>>>         Multiple profiles:                  no
>>>
>>> 		Data      Metadata  System
>>>     Id Path       RAID0     single    single   Unallocated
>>>     -- ---------- --------- --------- -------- -----------
>>>      1 /dev/sda10   1.00GiB   8.00MiB  1.00MiB     8.99GiB
>>>     -- ---------- --------- --------- -------- -----------
>>>        Total        1.00GiB   8.00MiB  1.00MiB     8.99GiB
>>>        Used       200.25MiB 352.00KiB 16.00KiB
>>>
>>>     # btrfs dev us .
>>>     /dev/sda10, ID: 1
>>>        Device size:            10.00GiB
>>>        Device slack:              0.00B
>>>        Data,RAID0/1:            1.00GiB
>>
>> Can we slightly enhance the output?
>> RAID0/1 really looks like a new profile now, even the "1" really means
>> the number of device.
>
> Do you have a concrete suggestion? This format was inspired by a
> discussion and suggested by users so I guess this is what people expect
> and I find it clear. It's also documented in manual page so if you think
> it's not clear or missing some important information, please let me
> know.

My idea may not be pretty though:

         Data,RAID0 (1 dev):            1.00GiB

As if we follow the existing pattern, it can be more confusing like:

         Data,RAID5/6

Thanks,
Qu