raid10 on three discs - few questions.

raid10 on three discs - few questions.

[Janek Kozicki]

Hi,

Maybe I'll buy three HDDs to put a raid10 on them. And get the total
capacity of 1.5 of a disc. 'man 4 md' indicates that this is possible
and should work.

I'm wondering - how a single disc failure is handled in such configuration?

1. does the array continue to work in a degraded state?

2. after the failure I can disconnect faulty drive, connect a new one,
   start the computer, add disc to array and it will sync automatically?


Question seems a bit obvious, but the configuration is, at least for
me, a bit unusual. This is why I'm asking. Anybody here tested such
configuration, has some experience?


3. Another thing - would raid10,far=2 work when three drives are used?
   Would it increase the read performance?

4. Would it be possible to later '--grow' the array to use 4 discs in
   raid10 ? Even with far=2 ?

thanks,
-- 
Janek Kozicki                                                         |

Re: raid10 on three discs - few questions.

[Neil Brown]

On Sunday February 3, janek_listy@wp.pl wrote:
> Hi,
> 
> Maybe I'll buy three HDDs to put a raid10 on them. And get the total
> capacity of 1.5 of a disc. 'man 4 md' indicates that this is possible
> and should work.
> 
> I'm wondering - how a single disc failure is handled in such configuration?
> 
> 1. does the array continue to work in a degraded state?

Yes.

> 
> 2. after the failure I can disconnect faulty drive, connect a new one,
>    start the computer, add disc to array and it will sync automatically?
> 

Yes.

> 
> Question seems a bit obvious, but the configuration is, at least for
> me, a bit unusual. This is why I'm asking. Anybody here tested such
> configuration, has some experience?
> 
> 
> 3. Another thing - would raid10,far=2 work when three drives are used?
>    Would it increase the read performance?

Yes.

> 
> 4. Would it be possible to later '--grow' the array to use 4 discs in
>    raid10 ? Even with far=2 ?
> 

No.

Well.... if by "later" you mean "in five years", then maybe.  But the
code doesn't currently exist.

NeilBrown

Re: raid10 on three discs - few questions.

[Janek Kozicki]

Neil Brown said:     (by the date of Mon, 4 Feb 2008 10:11:27 +1100)

wow, thanks for quick reply :)

> > 3. Another thing - would raid10,far=2 work when three drives are used?
> >    Would it increase the read performance?
> 
> Yes.

is far=2 the most I could do to squeeze every possible MB/sec
performance in raid10 on three discs ?

-- 
Janek Kozicki                                                         |

Re: raid10 on three discs - few questions.

[Jon Nelson]

On Feb 3, 2008 5:29 PM, Janek Kozicki <janek_listy@wp.pl> wrote:
> Neil Brown said:     (by the date of Mon, 4 Feb 2008 10:11:27 +1100)
>
> wow, thanks for quick reply :)
>
> > > 3. Another thing - would raid10,far=2 work when three drives are used?
> > >    Would it increase the read performance?
> >
> > Yes.
>
> is far=2 the most I could do to squeeze every possible MB/sec
> performance in raid10 on three discs ?

In my opinion, yes. It has sequential read characteristics that place
at /or better than/ raid0. Writing is slower, about the speed of a
single disk, give or take.  The other two raid10 layouts (near and
offset) are very close in performance to each other - nearly identical
for reading/writing.

-- 
Jon

Re: raid10 on three discs - few questions.

[Jon Nelson]

On Feb 3, 2008 5:29 PM, Janek Kozicki <janek_listy@wp.pl> wrote:
> Neil Brown said:     (by the date of Mon, 4 Feb 2008 10:11:27 +1100)
>
> wow, thanks for quick reply :)
>
> > > 3. Another thing - would raid10,far=2 work when three drives are used?
> > >    Would it increase the read performance?
> >
> > Yes.
>
> is far=2 the most I could do to squeeze every possible MB/sec
> performance in raid10 on three discs ?

In my opinion, yes. It has sequential read characteristics that place
at /or better than/ raid0. Writing is slower, about the speed of a
single disk, give or take.  The other two raid10 layouts (near and
offset) are very close in performance to each other - nearly identical
for reading/writing.

-- 
Jon

Re: raid10 on three discs - few questions.

[Jon Nelson]

On Feb 6, 2008 12:43 PM, Bill Davidsen <davidsen@tmr.com> wrote:

> Can you create a raid10 with one drive "missing" and add it later? I
> know, I should try it when I get a machine free... but I'm being lazy today.

Yes you can. With 3 drives, however, performance will be awful (at
least with layout far, 2 copies).

IMO raid10,f2 is a great balance of speed and redundancy.
it''s faster than raid5 for reading, about the same for writing. it's
even potentially faster than raid0 for reading, actually.
With 3 disks one should be able to get 3.0 times the speed of one
disk, or slightly more, and each stripe involves only *one* disk
instead of 2 as it does with raid5.

-- 
Jon

Re: raid10 on three discs - few questions.

[Bill Davidsen]

Neil Brown wrote:
> On Sunday February 3, janek_listy@wp.pl wrote:
>   
>> Hi,
>>
>> Maybe I'll buy three HDDs to put a raid10 on them. And get the total
>> capacity of 1.5 of a disc. 'man 4 md' indicates that this is possible
>> and should work.
>>
>> I'm wondering - how a single disc failure is handled in such configuration?
>>
>> 1. does the array continue to work in a degraded state?
>>     
>
> Yes.
>
>   
>> 2. after the failure I can disconnect faulty drive, connect a new one,
>>    start the computer, add disc to array and it will sync automatically?
>>
>>     
>
> Yes.
>
>   
>> Question seems a bit obvious, but the configuration is, at least for
>> me, a bit unusual. This is why I'm asking. Anybody here tested such
>> configuration, has some experience?
>>
>>
>> 3. Another thing - would raid10,far=2 work when three drives are used?
>>    Would it increase the read performance?
>>     
>
> Yes.
>
>   
>> 4. Would it be possible to later '--grow' the array to use 4 discs in
>>    raid10 ? Even with far=2 ?
>>
>>     
>
> No.
>
> Well.... if by "later" you mean "in five years", then maybe.  But the
> code doesn't currently exist.
>   

That's a reason to avoid raid10 for certain applications, then, and go 
with a more manual 1+0 or similar.

Can you create a raid10 with one drive "missing" and add it later? I 
know, I should try it when I get a machine free... but I'm being lazy today.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 

Re: raid10 on three discs - few questions.

[Bill Davidsen]

Jon Nelson wrote:
> On Feb 6, 2008 12:43 PM, Bill Davidsen <davidsen@tmr.com 
> <mailto:davidsen@tmr.com>> wrote:
>
>     Can you create a raid10 with one drive "missing" and add it later? I
>     know, I should try it when I get a machine free... but I'm being
>     lazy today.
>
>
> Yes you can. With 3 drives, however, performance will be awful (at 
> least with layout far, 2 copies).
>
Well, the question didn't include being fast. ;-)

But if he really wants to create the array now and be able to add to it 
later, it might still be useful, particularly if "later" is a small time 
like "when my other drive ships." Thanks for the input, I thought that 
was possible, but reading code isn't the same as testing.
> IMO raid10,f2 is a great balance of speed and redundancy.
> it''s faster than raid5 for reading, about the same for writing. it's 
> even potentially faster than raid0 for reading, actually.
> With 3 disks one should be able to get 3.0 times the speed of one 
> disk, or slightly more, and each stripe involves only *one* disk 
> instead of 2 as it does with raid5.

I have used raid10 swap on 3 or more drives fairly often. Other than the 
Fedora rescue CD not using the space until I start it manually, I find 
it really fast, and helpful for huge image work.

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark 

Re: raid10 on three discs - few questions.

[Neil Brown]

On Wednesday February 6, davidsen@tmr.com wrote:
> >
> >   
> >> 4. Would it be possible to later '--grow' the array to use 4 discs in
> >>    raid10 ? Even with far=2 ?
> >>
> >>     
> >
> > No.
> >
> > Well.... if by "later" you mean "in five years", then maybe.  But the
> > code doesn't currently exist.
> >   
> 
> That's a reason to avoid raid10 for certain applications, then, and go 
> with a more manual 1+0 or similar.

Not really.  You cannot reshape a raid0 either.

> 
> Can you create a raid10 with one drive "missing" and add it later? I 
> know, I should try it when I get a machine free... but I'm being lazy today.

Yes, but then the array would be degraded and a single failure could
destroy your data.

NeilBrown