From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: Problem with disk Date: Sun, 07 May 2006 07:56:33 +0900 Message-ID: <445D29A1.5000402@gmail.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from py-out-1112.google.com ([64.233.166.182]:59435 "EHLO py-out-1112.google.com") by vger.kernel.org with ESMTP id S932099AbWEFW5F (ORCPT ); Sat, 6 May 2006 18:57:05 -0400 Received: by py-out-1112.google.com with SMTP id x31so1138729pye for ; Sat, 06 May 2006 15:57:04 -0700 (PDT) In-Reply-To: Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Mark Hahn Cc: Ric Wheeler , David.Ronis@McGill.CA, linux-ide@vger.kernel.org Mark Hahn wrote: [--snip--] > I assume that the disk will indeed do writeback if left idle for a little > while. on machines where this is a real problem, I would start out by > waving relevant chickens like the following to give the best chance of > shutting down cleanly: > sync > blockdev --flushbufs > hdparm -W 0 > sleep 2 > hdparm -y > sleep 5 > halt -hp > > rather than _always_ suffering the penalty of disabled write cache, > especially on a single slow laptop drive... This is slightly OT as this thread is talking about normal power down but disabling writeback cache has its advantages. When you have power fluctation (e.g. power source fluctation or new device hot plugged and crappy PSU can't hold the voltage), the harddisk could briefly power down while other parts of system keep running. If the disk was under active FS writes, this ends up in inconsistencies between what the OS thinks the disk has and the disk actually has. Unfortunately, this can result in *massive* destruction of the filesystem. I lost my RAID-1 array earlier this year this way. The FS code systematically destroyed metadata of the filesystem and, on the following reboot, fsck did the final blow, I think. I ended up with 100+Gbytes of unorganized data and I had to recover data by grep + bvi. This is an extreme case but it shows turning off writeback has its advantages. After the initial stress & panic attack subsided, I tried to think about how to prevent such catastrophes, but there doesn't seem to be a good way. There's no way to tell 1. if the harddrive actually lost the writeback cache content 2. if so, how much it has lost. So, unless the OS halts the system everytime something seems weird with the disk, turning off writeback cache seems to be the only solution. -- tejun