From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Brown Subject: Re: Home desktop/server RAID upgrade Date: Sat, 31 May 2014 12:52:19 +0200 Message-ID: <5389B463.5020100@hesbynett.no> References: <20140530212907.0b00e8a3@netstation> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Mark Knecht , "L.M.J" Cc: Linux-RAID List-Id: linux-raid.ids On 30/05/14 22:14, Mark Knecht wrote: > On Fri, May 30, 2014 at 12:29 PM, L.M.J > wrote: >> Le Fri, 30 May 2014 12:04:07 -0700, Mark Knecht >> a =C3=A9crit : >> >>> In a RAID1 would a 3-drive Red RAID1 possibly be faster than the >>> 2-drive Se RAID1 and at the same time give me more safety? >> >> Just a question inside the question : how do you manager a RAID1 >> with 3 drives ? Maybe you're talking about RAID5 then ? > > OK, I'm no RAID expert but RAID1 is just drives in parallel right. 2 > drives, 3 drives, 4 drives, all holding exactly the same data. In > the case of a 3-drive RAID1 - if there is such a beast - I could > safely lose 2 drives. You ask a reasonable question though as maybe > the way this is actually done is 2 drives + a hot spare in the box > that gets sync'ed if and only if one drive fails. Not sure and maybe > I'm totally wrong about that. > > A 3-drive RAID5 would be 2 drives in series - in this case making > 6TB - and then the 3rd drive being the redundancy. In the case of a > 3-drive RAID5 I could safely lose 1 drive. > > In my case I don't need more than 3TB, so an option would be a > 3-drive RAID5 made out of 2TB drives which would give me 4TB but I > don't need the space as much as I want the redundancy and I think > RAID5 is slower than RAID1. Additionally some more mdadm RAID > knowledgeable people on other lists say Linux mdadm RAID1 would be > faster as it will get data from more than one drive at a time. (Or > possibly get data from which ever drive returns it the fastest. Not > sure.) > > I believe one good option if I wanted 4 physical drives would be > RAID10 but that's getting more complicated again which I didn't > really want to do. > > So maybe it is just 2 drives and the 3 drive version isn't even a > possibility? Could be. With 3 drives, you have several possibilities. Raid5 makes "stripes" across the three drives, with 2 parts holding dat= a=20 and one part holding parity to provide redundancy. Raid1 is commonly called "mirroring", because you get the same data on=20 each disk. md raid has no problem making a 3-way mirror, so that each=20 disk is identical. This gives you excellent redundancy, and you can=20 make three different reads in parallel - but writes have to go to each=20 disk, which can be a little slower than using 2 disks. It's not often=20 that people need that level of redundancy. Another option with md raid is the raid10 setups. For many uses, the=20 fastest arrangement is raid10,f2. This means there is two copies of al= l=20 your data (f3 would be three copies), with a "far" layout. With this arrangement, reads are striped across all three disks, which=20 is fast for large reads. Small reads can be handled in parallel. Most= =20 reads while be handled from the outer half of the disk, which is faster= =20 and needs less head movement - so reading is on average faster than a=20 raid0 on the same disks. Small writes are fast, but large writes=20 require quite a bit of head movement to get everything written twice to= =20 different parts of the disks. The "best" option always depends on your needs - how you want to access= =20 your files. A layout geared to fast striped reads of large files will=20 be poorer for parallel small writes, and vice versa. raid10,f2 is ofte= n=20 the best choice for a desktop or small system - but it is not very=20 flexible if you later want to add new disks or replace the disks with=20 bigger ones. md raid is flexible enough that it will even let you make a 3 disk raid= 6=20 array if you want - but a 3-way raid1 mirror will give you the same dis= k=20 space and much better performance. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html