From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Campbell Subject: Re: consistency detect Date: Mon, 11 Oct 2004 12:54:01 +0400 Sender: linux-raid-owner@vger.kernel.org Message-ID: <416A4A29.9080906@wasp.net.au> References: <1097465522.3111.24.camel@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1097465522.3111.24.camel@localhost.localdomain> To: mingz@ele.uri.edu Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Ming Zhang wrote: > I have a question on RAID error detect. hope somebody can help me to > find it out. thanks. > > take raid1 as an example, if one disk fail, raid 1 can detect the data > on disk is compromised and then reconstruct it using a spare disk. this > is straight forward. > > but if one request comes to raid1 and raid1 sends requests to both > disks, at this time, system reboots because power outage, system > crashes, or any other reason. then after system reboots, how raid 1 > detects which disk has consistent data? since before reboot, anything > can happen, data may in disk1 but not in disk2, or in disk2 but not in > disk1, or not in both disks, or already on both disks. > > how raid1 or other raid code deal with this? In short, it does not deal with it at all. RAID will deal with a disk failure, it has no guarantees about consistency on power failures, hard lockups or other catastrophic events. A UPS is cheap insurance against consistency issues in combination with a journalling filesystem. Brad