From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f169.google.com ([209.85.223.169]:39506 "EHLO mail-io0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726376AbeHQQEc (ORCPT ); Fri, 17 Aug 2018 12:04:32 -0400 Received: by mail-io0-f169.google.com with SMTP id l7-v6so6817492iok.6 for ; Fri, 17 Aug 2018 06:01:12 -0700 (PDT) Subject: Re: Experiences on BTRFS Dual SSD RAID 1 with outage of one SSD To: Roman Mamedov , Martin Steigerwald Cc: linux-btrfs@vger.kernel.org References: <2331408.nK7QgfhVWv@merkaba> <5591e89c-eb89-dc48-1dc6-dc46775c7817@gmail.com> <1598008.Z1BGxZKWFL@merkaba> <20180817175022.70cd084e@natsu> From: "Austin S. Hemmelgarn" Message-ID: <94de7bd8-1b35-ecbb-566a-86b5c9de06fc@gmail.com> Date: Fri, 17 Aug 2018 09:01:06 -0400 MIME-Version: 1.0 In-Reply-To: <20180817175022.70cd084e@natsu> Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 2018-08-17 08:50, Roman Mamedov wrote: > On Fri, 17 Aug 2018 14:28:25 +0200 > Martin Steigerwald wrote: > >>> First off, keep in mind that the SSD firmware doing compression only >>> really helps with wear-leveling. Doing it in the filesystem will help >>> not only with that, but will also give you more space to work with. >> >> While also reducing the ability of the SSD to wear-level. The more data >> I fit on the SSD, the less it can wear-level. And the better I compress >> that data, the less it can wear-level. > > Do not consider SSD "compression" as a factor in any of your calculations or > planning. Modern controllers do not do it anymore, the last ones that did are > SandForce, and that's 2010 era stuff. You can check for yourself by comparing > write speeds of compressible vs incompressible data, it should be the same. At > most, the modern ones know to recognize a stream of binary zeroes and have a > special case for that. All that testing write speeds forz compressible versus incompressible data tells you is if the SSD is doing real-time compression of data, not if they are doing any compression at all.. Also, this test only works if you turn the write-cache on the device off. Besides, you can't prove 100% for certain that any manufacturer who does not sell their controller chips isn't doing this, which means there are a few manufacturers that may still be doing it.