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

[waxhead <waxhead@dirtcellar.net>]

Hugo Mills wrote:
> On Sat, Jul 24, 2021 at 01:04:19PM +0200, waxhead wrote:
>> David Sterba wrote:
>>
>>> Maybe it's still too new so nobody is used to it and we've always had
>>> problems with the raid naming scheme anyway.
>>
>> Some years ago (2018) there was some talk about a new naming scheme
>> https://marc.info/?l=linux-btrfs&m=136286324417767
>>
>> While technically spot on I found Hugo's naming scheme difficult. It was
>> based on this idea:
>> numCOPIESnumSTRIPESnumPARITY
>>
>> 1CmS1P = Raid5 or 1 copy, max stripes, 1 parity.
>>
>> I also do not agree with the use of 'copy'. The Oxford dictionary defines
>> 'copy' as "a thing that is made to be the same as something else, especially
>> a document or a work of art"
>>
>> And while some might argue that copying something on disk from memory makes
>> it a copy, it ceases to be a copy once the memory contents is erased. I
>> therefore think that replicas is a far better terminology.
>>
>> I earlier suggested Rnum.Snum.Pnum as a naming scheme which I think is far
>> more readable so if I may dare to be as bold....
>>
>> SINGLE  = R0.S0.P0 (no replicas, no stripes (any device), no parity)
>> DUP     = R1.S1.P0 (1 replica, 1 stripe (one device), no parity)
>> RAID0   = R0.Sm.P0 (no replicas, max stripes, no parity)
>> RAID1   = R1.S0.P0 (1 replica, no stripes (any device), no parity)
>> RAID1c2 = R2.S0.P0 (2 replicas, no stripes (any device), no parity)
>> RAID1c3 = R3.S0.P0 (3 replicas, no stripes (any device), no parity)
>> RAID10  = R1.Sm.P0 (1 replica, max stripes, no parity)
>> RAID5   = R0.Sm.P1 (no replicas, max stripes, 1 parity)
>> RAID5   = R0.Sm.P2 (no replicas, max stripes, 2 parity)
>>
>> This (or Hugo's) type of naming scheme would also easily allow add more
>> exotic configuration such as S5 e.g. stripe over 5 devices in a 10 device
>> storage system which could increase throughput for certain workloads
>> (because it leaves half the storage devices "free" for other jobs)
>> A variant of RAID5 behaving like RAID10 would simply be R1.Sm.P1. Easy
>> peasy...  And just for the record , the old RAID terminology should of
>> course work for compatibility reasons, but ideally should not be advertised
>> at all.
>>
>> Sorry for completely derailing the topic, but I felt it was important to
>> bring up (and I admit to be overenthusiastic about it). :)
> 
>     I'd go along with that scheme, with one minor modification -- make
> the leading letters lower-case. The choice of lower-case letters in my
> original scheme was deliberate, as it breaks up the sequence and is
> much easier to pick out the most important parts (the numbers) from
> the mere positional markers (the letters).
> 
>     Also, the "M" (caps, because it's equivalent to the large numbers)
> in stripes wasn't for "max", but simply the conventional mathematical
> "m" -- some number acting as a limit to a counter (as in, "we have n
> copies with m stripes and p parity stripes").
> 
>     Hugo.
> 
Agree. Lowercase r0.s0.p0 / r1.sM.p2 is more readable indeed.
I insist on the dots between for separators as this would make possibly 
future fantasy things such as rmin-max e.g. r2-4.sM.p0 more readable.

(in my fantasy world: r2-6 would mean 6 replicas where all can 
automatically be deleted except 2 if the filesystem runs low on space. 
Would make parallel read potentially super fast as long as there is 
plenty of free space on the filesystem plus increase redundancy. Free 
space is wasted space (just like with memory so it might as well be used 
for something useful)