From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Pittman Subject: Re: RAID1 Mirroring question. Date: Tue, 06 Apr 2004 11:29:16 +1000 Sender: linux-raid-owner@vger.kernel.org Message-ID: <87brm5nbbn.fsf@enki.rimspace.net> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Mon, 5 Apr 2004, Mark Hahn wrote: >> We have a Linux Software RAID1 (mirroring). If there is a little >> error on one of the disks (such a little error that the kernel dosn't >> recognize it). There is a read request on the raid for a specific >> file. The output of one of the disks differ from the output of the >> other disk. (But there are no errors recognized by the kernel / fs / >> raid-driver. Only one inverted bit for example) What is RAID/MD >> doing? Are there checksums for the original file? [...] > then again, raid1 is a sort of ugly niche feature, IMO. how many > systems can afford two but not three disks? raid5 is not scary! *blink* Quite a few systems don't have the capacity for three-disk rather than two-disk RAID. My laptop, for example, could not add a third disk at all. Also, RAID-5 does not have *any* improvement over RAID-1 in terms of detecting this sort of single-disk unreported error. RAID-5 also has a higher cost in terms of CPU use - enough that it presents problems in a number of embedded system scenarios where RAID-1 is fine. Three disk RAID-1, on the other hand, does allow you to detect a single device failure by a "two to one vote" detection system... Daniel -- About the use of language: it is impossible to sharpen a pencil with a blunt ax. It is equally vain to try to do it with ten blunt axes instead. -- Edsger Dijkstra