From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f43.google.com ([209.85.218.43]:33156 "EHLO mail-oi0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756551AbcILMpz (ORCPT ); Mon, 12 Sep 2016 08:45:55 -0400 Received: by mail-oi0-f43.google.com with SMTP id y2so310942202oie.0 for ; Mon, 12 Sep 2016 05:45:55 -0700 (PDT) Subject: Re: Small fs To: Martin Steigerwald , Hugo Mills , linux-btrfs@vger.kernel.org References: <14309857.N6fGUh6qoJ@merkaba> <20160911194632.GF7138@carfax.org.uk> <34273168.IdA9o4YR3V@merkaba> From: "Austin S. Hemmelgarn" Message-ID: <7efbc874-d1e8-011e-20c8-f887aef8a4ac@gmail.com> Date: Mon, 12 Sep 2016 08:45:51 -0400 MIME-Version: 1.0 In-Reply-To: <34273168.IdA9o4YR3V@merkaba> Content-Type: text/plain; charset=windows-1252; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 2016-09-11 15:51, Martin Steigerwald wrote: > Am Sonntag, 11. September 2016, 19:46:32 CEST schrieb Hugo Mills: >> On Sun, Sep 11, 2016 at 09:13:28PM +0200, Martin Steigerwald wrote: >>> Am Sonntag, 11. September 2016, 16:44:23 CEST schrieb Duncan: >>>> * Metadata, and thus mixed-bg, defaults to DUP mode on a single-device >>>> filesystem (except on ssd where I actually still use it myself, and >>>> recommend it except for ssds that do firmware dedupe). In mixed-mode >>>> this means two copies of data as well, which halves the usable space. >>>> >>>> IOW, when using mixed-mode, which is recommended under a gig, and dup >>>> replication which is then the single-device default, effective usable >>>> space is **HALVED**, so 256 MiB btrfs size becomes 128 MiB usable. (!!) >>> >>> I donīt get this part. That is just *metadata* being duplicated, not the >>> actual *data* inside the files. Or am I missing something here? >> >> In mixed mode, there's no distinction: Data and metadata both use >> the same chunks. If those chunks are DUP, then both data and metadata >> are duplicated, and you get half the space available. > > In german Iīd say "autsch", in english according to pda.leo.org "ouch", to > this. > > Okay, I just erased using mixed mode as an idea from my mind altogether :). Until recently (when DUP mode for data on a single device got added), it did have advantages. If you're running on a single disk and need replication, it's a lot more efficient to just run a single partition in DUP mode than two in RAID1 mode. It also makes it a bit less likely to hit ENOSPC issues on small filesystems. > > Just like I think I will never use a BTRFS below 5 GiB. Well, with one > exception, maybe on the eMMC flash of the new Omnia Turris router that I hope > will arrive soon at my place. > FWIW, I don't use anything smaller than 8G unless I have to (with the notable exception of /boot), and when I do need to, I usually just use mixed mode because once you get that small, you need all the help you can get to keep from the whole FS being allocated to chunks.