Re: [PATCH 0/4] 3- and 4- copy RAID1

[Duncan <1i5t5.duncan@cox.net>]

Duncan posted on Wed, 18 Jul 2018 07:20:09 +0000 as excerpted:

>> As implemented in BTRFS, raid1 doesn't have striping.
> 
> The argument is that because there's only two copies, on multi-device
> btrfs raid1 with 4+ devices of equal size so chunk allocations tend to
> alternate device pairs, it's effectively striped at the macro level,
> with the 1 GiB device-level chunks effectively being huge individual
> device strips of 1 GiB.
> 
> At 1 GiB strip size it doesn't have the typical performance advantage of
> striping, but conceptually, it's equivalent to raid10 with huge 1 GiB
> strips/chunks.

I forgot this bit...

Similarly, multi-device single is regarded by some to be conceptually 
equivalent to raid0 with really huge GiB strips/chunks.

(As you may note, "the argument is" and "regarded by some" are distancing 
phrases.  I've seen the argument made on-list, but while I understand the 
argument and agree with it to some extent, I'm still a bit uncomfortable 
with it and don't normally make it myself, this thread being a noted 
exception tho originally I simply repeated what someone else already said 
in-thread, because I too agree it's stretching things a bit.  But it does 
appear to be a useful conceptual equivalency for some, and I do see the 
similarity.

Perhaps it's a case of coder's view (no code doing it that way, it's just 
a coincidental oddity conditional on equal sizes), vs. sysadmin's view 
(code or not, accidental or not, it's a reasonably accurate high-level 
description of how it ends up working most of the time with equivalent 
sized devices).)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman