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Old 09-25-2010, 08:04 PM
Ross Walker
 
Default Raid 10 questions...2 drive

On Sep 25, 2010, at 1:52 PM, Tom H <tomh0665@gmail.com> wrote:

> On Sat, Sep 25, 2010 at 11:48 AM, Ross Walker <rswwalker@gmail.com> wrote:
>> On Sep 25, 2010, at 9:11 AM, Christopher Chan <christopher.chan@bradbury.edu.hk> wrote:
>>> Jacob Bresciani wrote:
>>>> RAID10 requires at least 4 drives does it not?
>>>>
>>>> Since it's a strip set of mirrored disks, the smallest configuration I
>>>> can see is 4 disks, 2 mirrored pairs stripped.
>>>
>>> He might be referring to what he can get from the mdraid10 (i know, Neil
>>> Brown could have chosen a better name) which is not quite the same as
>>> nested 1+0. Doing it the nested way, you need at least 4 drives. Using
>>> mdraid10 is another story. Thanks Neil for muddying the waters!
>
>
>> True, but if you figure it out mdraid10 with 2 drives = raid1, you would need 3
>> drives to get the distributed copy feature of Neil's mdraid10.
>
> I had posted earlier (
> http://lists.centos.org/pipermail/centos/2010-September/099473.html )
> that mdraid10 with two drives is basically raid1 but that it has some
> mirroring options. In the "far layout" mirroring option (where,
> according to WP, "all the drives are divided into f sections and all
> the chunks are repeated in each section but offset by one device")
> reads are faster than mdraid1 or vanilla mdraid10 on two drives.

If you have any two copies of the same chunk on the same drive then redundancy is completely lost.

Therefore without loosing redundancy mdraid10 over two drives will have to be identical to raid1.

Reads on a raid1 can be serviced by either side of the mirror, I believe the policy is hard coded to round robin. I don't know if it is smart enough to distinguish sequential pattern from random and only service sequential reads from one side or not.

>> For true RAID10 support in Linux you create multiple mdraid1 physical
>> volumes, create a LVM volume group out of them and create logical
>> volumes that interleave between these physical volumes.
>
> Vanilla mdraid10 with four drives is "true raid10".

Well like you stated above that depends on the near or far layout pattern, you can get the same performance as a raid10 or better in certain workloads, but it really isn't a true raid10 in the sense that it isn't a stripe set of raid1s, but a distributed mirror set.

Now don't get me wrong I'm not saying it's not as good as a true raid10, in fact I believe it to be better as it provides way more flexibility and is a lot simpler of an implementation, but not really a raid10, but something completely new.

-Ross

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Old 09-25-2010, 08:11 PM
Miguel Medalha
 
Default Raid 10 questions...2 drive

> And don't do it that way.
>
> If you have a single drive failure with RAID 0+1 you've lost *all* of
> your redundancy - one more failure and you are dead. If you create two
> RAID1 sets and then strip them into a RAID0 you get pretty much the same
> performance and space efficiency characteristics, but if you have a
> drive failure you still have partial redundancy. You could actually take
> a *second* drive failure as long as it was in the other RAID1 pair. With
> 4 drives raid0+1 can only survive 1 drive failure. With 4 drives in raid
> 1+0 you can survive an average of 1.67 drive failures.

Indeed.

This article explains the odds of loosing data with RAID 1+0 vs 0+1:


Why is RAID 1+0 better than RAID 0+1?
http://www.aput.net/~jheiss/raid10/

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Old 09-25-2010, 08:14 PM
Tom Bishop
 
Default Raid 10 questions...2 drive

Thanks for all of the inputs...I finally came across a good article summarizing what I needed, looks like I am going to try to the f2 option and then do some testing vs the default n2 option.* I am building the array as we speak but it looks like building the f2 option will take 24hrs vs 2hrs for the n2 option....this is on 2 1TB hdd....


On Sat, Sep 25, 2010 at 3:04 PM, Ross Walker <rswwalker@gmail.com> wrote:

On Sep 25, 2010, at 1:52 PM, Tom H <tomh0665@gmail.com> wrote:



> On Sat, Sep 25, 2010 at 11:48 AM, Ross Walker <rswwalker@gmail.com> wrote:

>> On Sep 25, 2010, at 9:11 AM, Christopher Chan <christopher.chan@bradbury.edu.hk> wrote:

>>> Jacob Bresciani wrote:

>>>> RAID10 requires at least 4 drives does it not?

>>>>

>>>> Since it's a strip set of mirrored disks, the smallest configuration I

>>>> can see is 4 disks, 2 mirrored pairs stripped.

>>>

>>> He might be referring to what he can get from the mdraid10 (i know, Neil

>>> Brown could have chosen a better name) which is not quite the same as

>>> nested 1+0. Doing it the nested way, you need at least 4 drives. Using

>>> mdraid10 is another story. Thanks Neil for muddying the waters!

>

>

>> True, but if you figure it out mdraid10 with 2 drives = raid1, you would need 3

>> drives to get the distributed copy feature of Neil's mdraid10.

>

> I had posted earlier (

> http://lists.centos.org/pipermail/centos/2010-September/099473.html )

> that mdraid10 with two drives is basically raid1 but that it has some

> mirroring options. In the "far layout" mirroring option (where,

> according to WP, "all the drives are divided into f sections and all

> the chunks are repeated in each section but offset by one device")

> reads are faster than mdraid1 or vanilla mdraid10 on two drives.



If you have any two copies of the same chunk on the same drive then redundancy is completely lost.



Therefore without loosing redundancy mdraid10 over two drives will have to be identical to raid1.



Reads on a raid1 can be serviced by either side of the mirror, I believe the policy is hard coded to round robin. I don't know if it is smart enough to distinguish sequential pattern from random and only service sequential reads from one side or not.




>> For true RAID10 support in Linux you create multiple mdraid1 physical

>> volumes, create a LVM volume group out of them and create logical

>> volumes that interleave between these physical volumes.

>

> Vanilla mdraid10 with four drives is "true raid10".



Well like you stated above that depends on the near or far layout pattern, you can get the same performance as a raid10 or better in certain workloads, but it really isn't a true raid10 in the sense that it isn't a stripe set of raid1s, but a distributed mirror set.




Now don't get me wrong I'm not saying it's not as good as a true raid10, in fact I believe it to be better as it provides way more flexibility and is a lot simpler of an implementation, but not really a raid10, but something completely new.




-Ross



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http://lists.centos.org/mailman/listinfo/centos



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Old 09-25-2010, 08:15 PM
Miguel Medalha
 
Default Raid 10 questions...2 drive

> Mdraid10 actually allows for a 3 drive raid10 set. It isn't raid10 per say but a raid level based on distributing copies of chunks around the spindles for redundancy.

Isn't this what they call RAID 1e (RAID 1 Enhanced), which needs a
minimum of 3 drives?

This seems to me a much better name for it than calling it "RAID 10"...

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Old 09-25-2010, 09:01 PM
Nataraj
 
Default Raid 10 questions...2 drive

Miguel Medalha wrote:
>> Mdraid10 actually allows for a 3 drive raid10 set. It isn't raid10 per say but a raid level based on distributing copies of chunks around the spindles for redundancy.
>>
>
> Isn't this what they call RAID 1e (RAID 1 Enhanced), which needs a
> minimum of 3 drives?
>
> This seems to me a much better name for it than calling it "RAID 10"...
>
> _______________________________________________
> CentOS mailing list
> CentOS@centos.org
> http://lists.centos.org/mailman/listinfo/centos
>
Yes it is Raid-1E. This is explicitly documented in the link that
Karanbir provided...
http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10

Nataraj

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Old 09-25-2010, 09:09 PM
Ross Walker
 
Default Raid 10 questions...2 drive

On Sep 25, 2010, at 4:15 PM, Miguel Medalha <miguelmedalha@sapo.pt> wrote:

>
>> Mdraid10 actually allows for a 3 drive raid10 set. It isn't raid10 per say but a raid level based on distributing copies of chunks around the spindles for redundancy.
>
> Isn't this what they call RAID 1e (RAID 1 Enhanced), which needs a minimum of 3 drives?
>
> This seems to me a much better name for it than calling it "RAID 10"...

The raid1e type probably didn't exist when Neil Brown came up with the algorithm.

He should have patented it though...

Maybe he started out with the idea to create a raid10, but didn't want the complexity of managing sub-arrays so decided just to redistribute chunk copies instead and then it took off from there.

-Ross

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Old 09-25-2010, 09:46 PM
Miguel Medalha
 
Default Raid 10 questions...2 drive

> The raid1e type probably didn't exist when Neil Brown came up with the algorithm.

You are probably right.

> He should have patented it though...

Maybe...

> Maybe he started out with the idea to create a raid10, but didn't want the complexity of managing sub-arrays so decided just to redistribute chunk copies instead and then it took off from there.

Yes. I didn't want to sound harsh to him. I am VERY grateful for his
outstanding work.

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Old 09-26-2010, 02:39 AM
Tom H
 
Default Raid 10 questions...2 drive

On Sat, Sep 25, 2010 at 4:04 PM, Ross Walker <rswwalker@gmail.com> wrote:
> On Sep 25, 2010, at 1:52 PM, Tom H <tomh0665@gmail.com> wrote:
>> On Sat, Sep 25, 2010 at 11:48 AM, Ross Walker <rswwalker@gmail.com> wrote:
>>> On Sep 25, 2010, at 9:11 AM, Christopher Chan <christopher.chan@bradbury.edu.hk> wrote:
>>>> Jacob Bresciani wrote:
>>>>> RAID10 requires at least 4 drives does it not?
>>>>>
>>>>> Since it's a strip set of mirrored disks, the smallest configuration I
>>>>> can see is 4 disks, 2 mirrored pairs stripped.
>>>>
>>>> He might be referring to what he can get from the mdraid10 (i know, Neil
>>>> Brown could have chosen a better name) which is not quite the same as
>>>> nested 1+0. Doing it the nested way, you need at least 4 drives. Using
>>>> mdraid10 is another story. Thanks Neil for muddying the waters!
>>
>>
>>> True, but if you figure it out mdraid10 with 2 drives = raid1, you would need 3
>>> drives to get the distributed copy feature of Neil's mdraid10.
>>
>> I had posted earlier (
>> http://lists.centos.org/pipermail/centos/2010-September/099473.html )
>> that mdraid10 with two drives is basically raid1 but that it has some
>> mirroring options. In the "far layout" mirroring option (where,
>> according to WP, "all the drives are divided into f sections and all
>> the chunks are repeated in each section but offset by one device")
>> reads are faster than mdraid1 or vanilla mdraid10 on two drives.
>
> If you have any two copies of the same chunk on the same drive
> then redundancy is completely lost.
>
> Therefore without loosing redundancy mdraid10 over two drives
> will have to be identical to raid1.
>
> Reads on a raid1 can be serviced by either side of the mirror, I
> believe the policy is hard coded to round robin. I don't know if it is
> smart enough to distinguish sequential pattern from random and
> only service sequential reads from one side or not.
>
>>> For true RAID10 support in Linux you create multiple mdraid1 physical
>>> volumes, create a LVM volume group out of them and create logical
>>> volumes that interleave between these physical volumes.
>>
>> Vanilla mdraid10 with four drives is "true raid10".
>
> Well like you stated above that depends on the near or far layout pattern,
> you can get the same performance as a raid10 or better in certain
> workloads, but it really isn't a true raid10 in the sense that it isn't a stripe
> set of raid1s, but a distributed mirror set.
>
> Now don't get me wrong I'm not saying it's not as good as a true raid10,
> in fact I believe it to be better as it provides way more flexibility and is a
> lot simpler of an implementation, but not really a raid10, but something
> completely new.

You must've misunderstood me.

mdraid10 on two disks: it is raid1 but you have the option of
mirroring, for example, cylinder 24 on disk 1 with cylinder 48 on disk
2; the Wikipedia article says that it makes reads faster (I don't
understand why but that's a different story).

mdraid10 on four disks: it is true raid10 but you also have various
"--layout=" options.
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Old 09-26-2010, 04:34 PM
Tom Bishop
 
Default Raid 10 questions...2 drive

Thanks everyone for the input...I have decided to go with the f2 option, however the rebuild time seems to be taking quite a long time, almost 24hr...I have read that there are options for speeding this up but want to make sure that they are ok to do....has to do with setting the minimum speed limit...


# sysctl dev.raid.speed_limit_min

# sysctl dev.raid.speed_limit_max


On Sat, Sep 25, 2010 at 9:39 PM, Tom H <tomh0665@gmail.com> wrote:

On Sat, Sep 25, 2010 at 4:04 PM, Ross Walker <rswwalker@gmail.com> wrote:

> On Sep 25, 2010, at 1:52 PM, Tom H <tomh0665@gmail.com> wrote:

>> On Sat, Sep 25, 2010 at 11:48 AM, Ross Walker <rswwalker@gmail.com> wrote:

>>> On Sep 25, 2010, at 9:11 AM, Christopher Chan <christopher.chan@bradbury.edu.hk> wrote:

>>>> Jacob Bresciani wrote:

>>>>> RAID10 requires at least 4 drives does it not?

>>>>>

>>>>> Since it's a strip set of mirrored disks, the smallest configuration I

>>>>> can see is 4 disks, 2 mirrored pairs stripped.

>>>>

>>>> He might be referring to what he can get from the mdraid10 (i know, Neil

>>>> Brown could have chosen a better name) which is not quite the same as

>>>> nested 1+0. Doing it the nested way, you need at least 4 drives. Using

>>>> mdraid10 is another story. Thanks Neil for muddying the waters!

>>

>>

>>> True, but if you figure it out mdraid10 with 2 drives = raid1, you would need 3

>>> drives to get the distributed copy feature of Neil's mdraid10.

>>

>> I had posted earlier (

>> http://lists.centos.org/pipermail/centos/2010-September/099473.html )

>> that mdraid10 with two drives is basically raid1 but that it has some

>> mirroring options. In the "far layout" mirroring option (where,

>> according to WP, "all the drives are divided into f sections and all

>> the chunks are repeated in each section but offset by one device")

>> reads are faster than mdraid1 or vanilla mdraid10 on two drives.

>

> If you have any two copies of the same chunk on the same drive

> then redundancy is completely lost.

>

> Therefore without loosing redundancy mdraid10 over two drives

> will have to be identical to raid1.

>

> Reads on a raid1 can be serviced by either side of the mirror, I

> believe the policy is hard coded to round robin. I don't know if it is

> smart enough to distinguish sequential pattern from random and

> only service sequential reads from one side or not.

>

>>> For true RAID10 support in Linux you create multiple mdraid1 physical

>>> volumes, create a LVM volume group out of them and create logical

>>> volumes that interleave between these physical volumes.

>>

>> Vanilla mdraid10 with four drives is "true raid10".

>

> Well like you stated above that depends on the near or far layout pattern,

> you can get the same performance as a raid10 or better in certain

> workloads, but it really isn't a true raid10 in the sense that it isn't a stripe

> set of raid1s, but a distributed mirror set.

>

> Now don't get me wrong I'm not saying it's not as good as a true raid10,

> in fact I believe it to be better as it provides way more flexibility and is a

> lot simpler of an implementation, but not really a raid10, but something

> completely new.



You must've misunderstood me.



mdraid10 on two disks: it is raid1 but you have the option of

mirroring, for example, cylinder 24 on disk 1 with cylinder 48 on disk

2; the Wikipedia article says that it makes reads faster (I don't

understand why but that's a different story).



mdraid10 on four disks: it is true raid10 but you also have various

"--layout=" options.

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Old 09-27-2010, 10:39 AM
John Doe
 
Default Raid 10 questions...2 drive

From: Tom Bishop <bishoptf@gmail.com>
>Thanks everyone for the input...I have decided to go with the f2 option, however
>
>the rebuild time seems to be taking quite a long time, almost 24hr...I have read
>
>that there are options for speeding this up but want to make sure that they are

>ok to do....has to do with setting the minimum speed limit...
>
># sysctl dev.raid.speed_limit_min
># sysctl dev.raid.speed_limit_max

It just sets how "aggressive" the reconstruction will be (depending on server
load, etc)...
If you set it low, the reconstruction will be slow, but the server fast.
If you set it high, the reconstruction will be fast, but the server slow.

JD



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