Re: Home desktop/server RAID upgrade

[David Brown <david.brown@hesbynett.no>]

On 30/05/14 22:14, Mark Knecht wrote:
> On Fri, May 30, 2014 at 12:29 PM, L.M.J <linuxmasterjedi@free.fr>
> wrote:
>> Le Fri, 30 May 2014 12:04:07 -0700, Mark Knecht
>> <markknecht@gmail.com> a écrit :
>>
>>> 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 data 
and one part holding parity to provide redundancy.

Raid1 is commonly called "mirroring", because you get the same data on 
each disk.  md raid has no problem making a 3-way mirror, so that each 
disk is identical.  This gives you excellent redundancy, and you can 
make three different reads in parallel - but writes have to go to each 
disk, which can be a little slower than using 2 disks.  It's not often 
that people need that level of redundancy.

Another option with md raid is the raid10 setups.  For many uses, the 
fastest arrangement is raid10,f2.  This means there is two copies of all 
your data (f3 would be three copies), with a "far" layout.

<http://en.wikipedia.org/wiki/Linux_MD_RAID_10#LINUX-MD-RAID-10>

With this arrangement, reads are striped across all three disks, which 
is fast for large reads.  Small reads can be handled in parallel.  Most 
reads while be handled from the outer half of the disk, which is faster 
and needs less head movement - so reading is on average faster than a 
raid0 on the same disks.  Small writes are fast, but large writes 
require quite a bit of head movement to get everything written twice to 
different parts of the disks.

The "best" option always depends on your needs - how you want to access 
your files.  A layout geared to fast striped reads of large files will 
be poorer for parallel small writes, and vice versa.  raid10,f2 is often 
the best choice for a desktop or small system - but it is not very 
flexible if you later want to add new disks or replace the disks with 
bigger ones.

md raid is flexible enough that it will even let you make a 3 disk raid6 
array if you want - but a 3-way raid1 mirror will give you the same disk 
space and much better performance.


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