From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from [195.159.176.226] ([195.159.176.226]:43727 "EHLO blaine.gmane.org" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1726708AbeGRJS5 (ORCPT ); Wed, 18 Jul 2018 05:18:57 -0400 Received: from list by blaine.gmane.org with local (Exim 4.84_2) (envelope-from ) id 1ffi03-0003DG-7O for linux-btrfs@vger.kernel.org; Wed, 18 Jul 2018 10:39:55 +0200 To: linux-btrfs@vger.kernel.org From: Duncan <1i5t5.duncan@cox.net> Subject: Re: [PATCH 0/4] 3- and 4- copy RAID1 Date: Wed, 18 Jul 2018 08:39:48 +0000 (UTC) Message-ID: References: <9945d460-99b5-a927-a614-c797bbc7862d@dirtcellar.net> <793d8ec3-7934-ea60-521d-7a039c9f1ce9@libero.it> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: 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