From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mr001msb.fastweb.it ([85.18.95.85]:57540 "EHLO mr001msb.fastweb.it" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753105AbdKORBT (ORCPT ); Wed, 15 Nov 2017 12:01:19 -0500 Received: from ceres.assyoma.it (93.63.55.57) by mr001msb.fastweb.it (8.5.140.05) id 59E3E61F015F8988 for linux-xfs@vger.kernel.org; Wed, 15 Nov 2017 18:01:17 +0100 Subject: Disabling barriers on NVC-backed HDD MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Wed, 15 Nov 2017 18:01:17 +0100 From: Gionatan Danti Message-ID: <8b91c685e344bdc5d084b11fab1d50af@assyoma.it> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: linux-xfs@vger.kernel.org Cc: g.danti@assyoma.it Hi list, I'm not sure this is the correct list to post my question; if so, feel free to ignore this message. On XFS FAQs I (rightfully) read that barrier should be *always* enabled (and they are, by default), unless a write-loss protected writeback cache is used at the RAID card level. So far, so good. Newer HDDs has themselves have a non-volatile cache (NVC) of some sort, basically: - DRAM + eMCL: in a powerloss event, the DRAM cache is immediately dumped on the eMLC flash; - DRAM + NOR flash: NOR mirrors a (small) portion of the DRAM cache, used for write acceleration purpose; - dedicated "dump" areas on the disks: they effectively mirror a portion of the DRAM cache. My question is: do you think it is safe to disable barriers, both for XFS and in general terms, on these disks? Or they should be considered as the same "dumb" unprotected DRAM caches found on classical HDD? From a side, these *are* powerloss-protected caches. Problem is that all these power-protection schemes are considered "secret sauce / trade secret" by HDD vendors and, for this reason, there are very little (if any) informations on their inner working. Regards. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: g.danti@assyoma.it - info@assyoma.it GPG public key ID: FF5F32A8