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Old 09-25-2010, 02:27 AM
Tom Bishop
 
Default Raid 10 questions...2 drive

I have been reading lots of stuff but trying to find out if a raid10 2drive setup is any better/worse than a normal raid 1 setup....I have to 1Tb drives for my data and a seperate system drive, I am only interested in doing raid on my data...



So i setup my initial test like this....

mdadm -v --create /dev/md0 --chunk 1024 --level=raid10 --raid-devices=2 /dev/sdb1 /dev/sdc1


I have also read about near and far but was going to play with this and was wondering if anyone had any insights for 2 drives setup...Thanks...

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Old 09-25-2010, 02:50 AM
Digimer
 
Default Raid 10 questions...2 drive

On 10-09-24 10:27 PM, Tom Bishop wrote:
> I have been reading lots of stuff but trying to find out if a raid10
> 2drive setup is any better/worse than a normal raid 1 setup....I have to
> 1Tb drives for my data and a seperate system drive, I am only interested
> in doing raid on my data...
>
>
> So i setup my initial test like this....
>
> mdadm -v --create /dev/md0 --chunk 1024 --level=raid10 --raid-devices=2
> /dev/sdb1 /dev/sdc1
>
>
> I have also read about near and far but was going to play with this and
> was wondering if anyone had any insights for 2 drives setup...Thanks...

Raid 10 requires 4 drives. First you would make two RAID 0 arrays, then
create a third array that is RAID 1 using the two RAID 0 arrays for it's
devices.

With only two drives, your option is RAID 1 (mirroring - proper
redundancy) or RAID 0 (striping only - lose one drive and you lose *all*
data).

--
Digimer
E-Mail: linux@alteeve.com
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Old 09-25-2010, 02:50 AM
Jacob Bresciani
 
Default Raid 10 questions...2 drive

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.

On Fri, Sep 24, 2010 at 7:27 PM, Tom Bishop <bishoptf@gmail.com> wrote:


I have been reading lots of stuff but trying to find out if a raid10 2drive setup is any better/worse than a normal raid 1 setup....I have to 1Tb drives for my data and a seperate system drive, I am only interested in doing raid on my data...





So i setup my initial test like this....

mdadm -v --create /dev/md0 --chunk 1024 --level=raid10 --raid-devices=2 /dev/sdb1 /dev/sdc1


I have also read about near and far but was going to play with this and was wondering if anyone had any insights for 2 drives setup...Thanks...




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--
Jacob BrescianiLinux Systems AdministratorAdvanced Ecommerce Research Systems / TerapeakCell: 250 418-5412


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Old 09-25-2010, 02:53 AM
Jacob Bresciani
 
Default Raid 10 questions...2 drive

On Fri, Sep 24, 2010 at 7:50 PM, Digimer <linux@alteeve.com> wrote:


On 10-09-24 10:27 PM, Tom Bishop wrote:

> I have been reading lots of stuff but trying to find out if a raid10

> 2drive setup is any better/worse than a normal raid 1 setup....I have to

> 1Tb drives for my data and a seperate system drive, I am only interested

> in doing raid on my data...

>

>

> So i setup my initial test like this....

>

> mdadm -v --create /dev/md0 --chunk 1024 --level=raid10 --raid-devices=2

> /dev/sdb1 /dev/sdc1

>

>

> I have also read about near and far but was going to play with this and

> was wondering if anyone had any insights for 2 drives setup...Thanks...



Raid 10 requires 4 drives. First you would make two RAID 0 arrays, then

create a third array that is RAID 1 using the two RAID 0 arrays for it's

devices.



This would be a RAID 0+1, stripped set's mirror together.*RAID 1+0 is mirrored disk sets stripped together.
http://en.wikipedia.org/wiki/RAID#Nested_.28hybrid.29_RAID



With only two drives, your option is RAID 1 (mirroring - proper

redundancy) or RAID 0 (striping only - lose one drive and you lose *all*

data).



--

Digimer

E-Mail: * * * * linux@alteeve.com

AN!Whitepapers: http://alteeve.com

Node Assassin: *http://nodeassassin.org

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--
Jacob BrescianiLinux Systems AdministratorAdvanced Ecommerce Research Systems / TerapeakCell: 250 418-5412

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Old 09-25-2010, 04:28 AM
tomh0665
 
Default Raid 10 questions...2 drive

On Fri, Sep 24, 2010 at 10:50 PM, Digimer <linux@alteeve.com> wrote:
> On 10-09-24 10:27 PM, Tom Bishop wrote:
>> I have been reading lots of stuff but trying to find out if a raid10
>> 2drive setup is any better/worse than a normal raid 1 setup....I have to
>> 1Tb drives for my data and a seperate system drive, I am only interested
>> in doing raid on my data...
>>
>> So i setup my initial test like this....
>>
>> mdadm -v --create /dev/md0 --chunk 1024 --level=raid10 --raid-devices=2
>> /dev/sdb1 /dev/sdc1
>>
>> I have also read about near and far but was going to play with this and
>> was wondering if anyone had any insights for 2 drives setup...Thanks...
>
> Raid 10 requires 4 drives. First you would make two RAID 0 arrays, then
> create a third array that is RAID 1 using the two RAID 0 arrays for it's
> devices.
>
> With only two drives, your option is RAID 1 (mirroring - proper
> redundancy) or RAID 0 (striping only - lose one drive and you lose *all*
> data).

mdraid does offer a 2-disk raid10 option but it is basically raid1
with some extra mirroring options:
http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10

You can specify the layout options with "--layout". From the man page:

<begin>
The layout options for RAID10 are one of 'n', 'o' or 'p' followed by a
small number. The default is 'n2'.

n signals 'near' copies. Multiple copies of one data block are at
similar offsets in different devices.

o signals 'offset' copies. Rather than the chunks being duplicated
within a stripe, whole stripes are duplicated but are rotated by one
device so duplicate blocks are on different devices. Thus subsequent
copies of a block are in the next drive, and are one chunk further
down.

f signals 'far' copies (multiple copies have very different offsets).
See md(4) for more detail about 'near' and 'far'.

The number is the number of copies of each datablock. 2 is normal, 3
can be useful. This number can be at most equal to the number of
devices in the array. It does not need to divide evenly into that
number (e.g. it is perfectly legal to have an 'n2' layout for an array
with an odd number of devices).
</end>
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Old 09-25-2010, 12:06 PM
Benjamin Franz
 
Default Raid 10 questions...2 drive

On 09/24/2010 07:50 PM, Digimer wrote:
> Raid 10 requires 4 drives. First you would make two RAID 0 arrays, then
> create a third array that is RAID 1 using the two RAID 0 arrays for it's
> devices.
>
> With only two drives, your option is RAID 1 (mirroring - proper
> redundancy) or RAID 0 (striping only - lose one drive and you lose *all*
> data).
>
>

That's 0+1 not 1+0.

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.

--
Benjamin Franz
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Old 09-25-2010, 12:43 PM
Karanbir Singh
 
Default Raid 10 questions...2 drive

On 09/25/2010 01:06 PM, Benjamin Franz wrote:
> 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
>

Things get a bit 'grey' with the mdraid10 and extentions, look at :
http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10
for an overview.

- KB
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Old 09-25-2010, 01:11 PM
Christopher Chan
 
Default Raid 10 questions...2 drive

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!
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Old 09-25-2010, 03:48 PM
Ross Walker
 
Default Raid 10 questions...2 drive

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.

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.

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.

This can give you the ability to extend a LVM RAID10 VG by adding RAID10 PVs to the VG. Unfortunately there isn't a resilver feature to LVM so you need to create a new LV to stripe it across all the members afterward, so leave room in the VG to do that.

-Ross

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Old 09-25-2010, 05:52 PM
Tom H
 
Default Raid 10 questions...2 drive

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.


> 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".
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