From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: possibly silly question (raid failover) Date: Tue, 01 Nov 2011 23:00:01 +0100 Message-ID: References: <4EAF3F78.5060900@meetinghouse.net> <4EAFEE95.6070608@meetinghouse.net> <4EAFF636.6060904@anonymous.org.uk> <4EB052E6.4050400@meetinghouse.net> <20111101212023.GA20565@cthulhu.home.robinhill.me.uk> <4EB06561.8090706@meetinghouse.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4EB06561.8090706@meetinghouse.net> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 01/11/11 22:32, Miles Fidelman wrote: > Robin Hill wrote: >> On Tue Nov 01, 2011 at 04:13:26 -0400, Miles Fidelman wrote: >> >>> David Brown wrote: >>>> No, md RAID10 does /not/ offer more redundancy than RAID1. You are >>>> right that md RAID10 offers more than RAID1 (or traditional RAID0 over >>>> RAID1 sets) - but it is a convenience and performance benefit, not a >>>> redundancy benefit. In particular, it lets you build RAID10 from any >>>> number of disks, not just two. And it lets you stripe over all disks, >>>> improving performance for some loads (though not /all/ loads - if you >>>> have lots of concurrent small reads, you may be faster using plain >>>> RAID1). >>> wasn't suggesting that it does - just that it does things differently >>> than normal raid 1+0 - for example, by doing mirroring and striping as a >>> unitary operation, it works across odd number of drives - it also (I >>> think) allows for more than 2 copies of a block (not completely clear >>> how many copies of a block would be made if you specified a 16 drive >>> array) - sort of what I'm wondering here >>> >> By default it'll make 2 copies, regardless how many devices are in the >> array. You can specify how many copies you want though, so -n3 will give >> you a near configuration with 3 copies, -n4 for four copies, etc. >> >> > cool, so with 16 drives, and say -n6 or -n8, and a far configuration - > that gives a pretty good level of resistance to multi-disk failures, as > well as an entire node failure (taking out 4 drives) You are aware, of course, that if you take your 16 drives and use "-n8", you will get a total disk space equivalent to two drives. It would be very resistant to drive failures, but /very/ poor space efficiency. It would also be very fast for reads, but very slow for writes (as everything must be written 8 times). It's your choice - md is very flexible. But I think an eight-way mirror would be considered somewhat unusual. > > which then leaves the question of whether the md driver, itself, can be > failed over from one node to another > > Thanks! > > Miles >