Re: [gentoo-user] RAID 1 vs RAID 0 - Read perfonmance

[Kerin Millar]

On 24/02/2014 06:27, Facundo Curti wrote:
> Hi. I am again, with a similar question to previous.
>
> I want to install RAID on SSD's.
>
> Comparing THEORETICALLY, RAID0 (stripe) vs RAID1 (mirrior). The
> performance would be something like this:
>
> n= number of disks
>
> reads:
>    raid1: n*2
>    raid0: n*2
>
> writes:
>    raid1: n
>    raid0: n*2
>
> But, in real life, the reads from raid 0 doesn't work at all, because if
> you use "chunk size" from 4k, and you need to read just 2kb (most binary
> files, txt files, etc..). the read speed should be just of n.

While the workload does matter, that's not really how it works. Be aware 
that Linux implements read-ahead (defaulting to 128K):-

# blockdev --getra /dev/sda
256

That's enough to populate 32 pages in pagecache, given that PAGESIZE is 
4K on i386/am64.

>
> On the other side, I read over the net, that kernel don't support
> multithread reads on raid1. So, the read speed will be just n. Always.
> ¿It is true?

No, it is not true. Read balancing is implemented in RAID-1.

>
> Anyway, my question is. ¿Who have the best read speed for the day to
> day? I'm not asking about reads off large files. I'm just asking in the
> "normal" use. Opening firefox, X, regular files, etc..

For casual usage, it shouldn't make any difference.

>
> I can't find the guide definitive. It allways are talking about
> theoretically performance, or about "real life" but without benchmarks
> or reliable data.
>
> Having a RAID0 with SSD, and following [2] on "SSD Stripe Optimization"
> should I have the same speed as an RAID1?

I would highly recommend conducting your own benchmarks. I find sysbench 
to be particularly useful.


>
> My question is because i'm between. 4 disks raid1, or RAID10 (I want
> redundancy anyway..). And as "raid 10" = 1+ 0. I need to know raid0
> performance to take a choice... I don't need write speed, just read.

In Linux, RAID-10 is not really nested because the mirroring and 
striping is fully integrated. If you want the best read performance with 
RAID-10 then the "far" layout is supposed to be the best [1].

Here is an example of how to choose this layout:

# mdadm -C /dev/md0 -n 4 -l 10 -p f2 /dev/sda /dev/sdb /dev/sdc /dev/sdd

Note, however, that the far layout will exhibit worse performance than 
the "near" layout if the array is in a degraded state. Also, it 
increases seek time in random/mixed workloads but this should not matter 
if you are using SSDs.

--Kerin

[1] http://neil.brown.name/blog/20040827225440