Re: Raid 1 vs Raid 10 single thread performance

[NeilBrown <neilb@suse.de>]

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On Wed, 10 Sep 2014 23:24:11 +0200 Bostjan Skufca <bostjan@a2o.si> wrote:

> Hi,
> 
> I have a simple question:
> - Where is the code that is used for actual RAID 10 creation? In
> kernel or in mdadm?

Depends on exactly what you mean ... probably in mdadm.

> 
> 
> Explanation:
> 
> I was dissatisfied with single-threaded RAID 1 sequential read
> performance (basically boils down to the speed of one disk). I figured
> that instead of using level 1 I could create RAID level 10 and use two
> equally-sized partitions on each drive (instead of one).
> 
> It turns out that if array is created properly, it is capable of
> sequential reads at almost 2x single device speed, as expected (on
> SSD!) and what would anyone expect from ordinary RAID 1.
> 
> What does "properly" actually mean?
> I was doing some benchmarks with various raid configurations and
> figured out that the order of devices submitted to creation command is
> significant. It also makes raid10 created in such mode reliable or
> unreliable to a device failure (not partition failure, device failure,
> which means that two raid underlying devices fail at once).

I don't think you've really explained what "properly" means.  How exactly do
you get better throughput?

If you want double-speed single-thread throughput on 2 devices, then create a
2-device RAID10 with "--layout=f2".



> 
> Sum:
> - if such array is created properly, it has redundancy in place and
> performs as expected
> - if not, it performs as raid1 and fails with one physical disk failure
> 
> I am trying to find the code responsible for creation of RAID 10 in
> order to try and make it more inteligent about where to place RAID 10
> parts if it gets a list of devices to use, and some of those devices
> are on the same physical disks.

mdadm uses the devices in the order that you list them.


> 
> Thanks for hints,
> b.
> 

NeilBrown


> 
> 
> PS: More details about testing is available here, but be warned, it is
> still a bit hectic to read:
> http://blog.a2o.si/2014/09/07/linux-software-raid-why-you-should-always-use-raid-10-instead-of-raid-1/
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