RAID 10 / 2 Devices Layout question

RAID 10 / 2 Devices Layout question

[Paul Roland]

Hello, quick one I guess sorry if I am messing this email with such a
newbie question.
I have two SSDs, and I would like to use md/raid10 (2devices) for
performance reasons.
The question since I was only able to find straight answers on more
than 2 devices:

Is using near instead of N2 somehow less secure than F2?
I know there is a mirrors and then stripe but with N2 I suspect a very
tiny chance of data to be written on same physical disc. In a 4disk
that is easy to understand or anticipate but here I am not sure which
to use.

Thank you

Re: RAID 10 / 2 Devices Layout question

[Phil Turmel]

On 06/26/2016 01:20 PM, Paul Roland wrote:

> I have two SSDs, and I would like to use md/raid10 (2devices) for
> performance reasons.

Ok, that's reasonable.

> Is using near instead of N2 somehow less secure than F2?

{ First, "near" == "near2" == "n2".  Linux raid10 defaults to two copies
of each data block.  "near3" == "n3" would be three copies of each data
block.  Et cetera. }

No.  The far layouts have exactly the same redundancy as the
corresponding near or offset layout.  The far layouts are intended to
provide a slight performance boost based on the difference in linear
head velocity on spinning disks between the outer edge and the hub.  It
has no advantage on SSDs.

The near layouts are equivalent to mirrors when using the same number of
disks as number of copies.  The near layouts are equivalent to mirrors
of raid0 stripes when the number of disks is a multiple of the number of
copies.

HTH,

Phil

Re: RAID 10 / 2 Devices Layout question

[Roman Mamedov]

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On Sun, 26 Jun 2016 13:34:05 -0400
Phil Turmel <philip@turmel.org> wrote:

> On 06/26/2016 01:20 PM, Paul Roland wrote:
> 
> > I have two SSDs, and I would like to use md/raid10 (2devices) for
> > performance reasons.
> 
> Ok, that's reasonable.

Maybe I'm missing something, but how is a RAID10 of just two devices is
considered reasonable without any questions or explanation? What is the actual
layout that is expected here, and what benefits (or even differences) does it
have compared to RAID0? I thought you need at least 3 devices (and h/w RAID
controllers might even require 4) for RAID10 to start making sense.

-- 
With respect,
Roman

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Re: RAID 10 / 2 Devices Layout question

[keld]

On Tue, Jun 28, 2016 at 02:25:02PM +0500, Roman Mamedov wrote:
> On Sun, 26 Jun 2016 13:34:05 -0400
> Phil Turmel <philip@turmel.org> wrote:
> 
> > On 06/26/2016 01:20 PM, Paul Roland wrote:
> > 
> > > I have two SSDs, and I would like to use md/raid10 (2devices) for
> > > performance reasons.
> > 
> > Ok, that's reasonable.
> 
> Maybe I'm missing something, but how is a RAID10 of just two devices is
> considered reasonable without any questions or explanation? What is the actual
> layout that is expected here, and what benefits (or even differences) does it
> have compared to RAID0? I thought you need at least 3 devices (and h/w RAID
> controllers might even require 4) for RAID10 to start making sense.

I think your understanding is a common misunderstanding of the Linux raid10.
There is more on this subject on:

https://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10

In short: yes Linux MD RAID10 is a different beast than standard RAID 1+0,
It can work with improved performance with just 2 disks, and even perform
like RAID0 for the Linux MD RAID10 far layout

See more on https://raid.wiki.kernel.org/index.php/Performance

best regards
Keld

Re: RAID 10 / 2 Devices Layout question

[Phil Turmel]

On 06/28/2016 05:25 AM, Roman Mamedov wrote:
> On Sun, 26 Jun 2016 13:34:05 -0400
> Phil Turmel <philip@turmel.org> wrote:
> 
>> On 06/26/2016 01:20 PM, Paul Roland wrote:
>>
>>> I have two SSDs, and I would like to use md/raid10 (2devices) for
>>> performance reasons.
>>
>> Ok, that's reasonable.
> 
> Maybe I'm missing something, but how is a RAID10 of just two devices is
> considered reasonable without any questions or explanation? What is the actual
> layout that is expected here, and what benefits (or even differences) does it
> have compared to RAID0? I thought you need at least 3 devices (and h/w RAID
> controllers might even require 4) for RAID10 to start making sense.

Paul made a statement that isn't obviously a problem -- raid10 on SSDs
makes a blistering fast general purpose redundant setup.  I use
precisely this setup for my boot and OS devices.  I'd do it for my
databases too if I could afford it.  (Can't justify it for my
performance needs -- darn.)

If Paul had asked whether or not to use raid10, I'd have had follow up
questions on his application.