RAID0 on disks with different sizes

RAID0 on disks with different sizes

[rj]

Hi,

How does MD RAID0 layer support disks with different sizes?
1. Does it take the size of smallest disk and do the striping
accordingly?  This way there would be wastage of space in bigger
disks.
Or
2. Does it stripe across all the space of disks? This way, if striping
is not possible for a disk because it has exhausted the space,
striping will continue or remaining devices (although with
compromising performance for these stripes because they have one disk
less).

What approach md chooses? Also, what approach is taken for RAID5 in such cases?

Thanks in advance.

Re: RAID0 on disks with different sizes

[Joachim Otahal]

rj schrieb:
> Hi,
>
> How does MD RAID0 layer support disks with different sizes?
>    
All PARTITIONS to create a RAID0 device should be the same size, else 
the smallest is the one that counts.
If you have 250GB+500GB+750GB:
Create RAID0 with threee drive using the space from 0-250 GB, then a 
RAID1 with the space from 251 to 500, and the last one just normal (or 
linear). You can combine them with LVM.
You _could_ create 6 partitions with 250 GB and create a RAID0 across 
all of them, but that will hurt the performance a lot.
Best for all in one RAID is possible with "linear" = JBOD.

> 1. Does it take the size of smallest disk and do the striping
> accordingly?  This way there would be wastage of space in bigger
> disks.
> Or
> 2. Does it stripe across all the space of disks? This way, if striping
> is not possible for a disk because it has exhausted the space,
> striping will continue or remaining devices (although with
> compromising performance for these stripes because they have one disk
> less).
>
> What approach md chooses? Also, what approach is taken for RAID5 in such cases?
>    
md does not choose on it's own, you set it up. When using disks like the 
example above: RAID 5 across the spave of 0-250 GB.

> Thanks in advance.
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>
>    

Re: RAID0 on disks with different sizes

[Beolach]

On Sun, Mar 14, 2010 at 23:26, rj <ratnadeep@gmail.com> wrote:
> Hi,
>
> How does MD RAID0 layer support disks with different sizes?
> 1. Does it take the size of smallest disk and do the striping
> accordingly?  This way there would be wastage of space in bigger
> disks.
> Or
> 2. Does it stripe across all the space of disks? This way, if striping
> is not possible for a disk because it has exhausted the space,
> striping will continue or remaining devices (although with
> compromising performance for these stripes because they have one disk
> less).
>
> What approach md chooses? Also, what approach is taken for RAID5 in such cases?
>

MD RAID0 follows #2; 4/5/6 follow #1.  From the man page for md:

   RAID0
<snip>
       If devices in the array are not all the same size, then once the small‐
       est device has been  exhausted,  the  RAID0  driver  starts  collecting
       chunks  into smaller stripes that only span the drives which still have
       remaining space.

   RAID1
<snip>
       All devices in a RAID1 array should be the same size.  If they are not,
       then  only the amount of space available on the smallest device is used
       (any extra space on other devices is wasted).
<snip>

   RAID4
<snip>
       Unlike RAID0, RAID4 also requires that all stripes span all drives,  so
       extra space on devices that are larger than the smallest is wasted.
<snip>

My understanding is that this is why MD RAID0 does not yet support
--grow, while MD RAID456 does - since they take different striping
approaches, it's much harder to reuse the --grow algorithm.


Good luck,
Conway S. Smith
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Re: RAID0 on disks with different sizes

[rj]

Okay, not it is confusing.. having two different answers to this
question :) Which answer is correct?
RAID0 takes approach 1 or approach2?

On Mon, Mar 15, 2010 at 1:28 PM, Beolach <beolach@gmail.com> wrote:
> On Sun, Mar 14, 2010 at 23:26, rj <ratnadeep@gmail.com> wrote:
>> Hi,
>>
>> How does MD RAID0 layer support disks with different sizes?
>> 1. Does it take the size of smallest disk and do the striping
>> accordingly?  This way there would be wastage of space in bigger
>> disks.
>> Or
>> 2. Does it stripe across all the space of disks? This way, if striping
>> is not possible for a disk because it has exhausted the space,
>> striping will continue or remaining devices (although with
>> compromising performance for these stripes because they have one disk
>> less).
>>
>> What approach md chooses? Also, what approach is taken for RAID5 in such cases?
>>
>
> MD RAID0 follows #2; 4/5/6 follow #1.  From the man page for md:
>
>   RAID0
> <snip>
>       If devices in the array are not all the same size, then once the small‐
>       est device has been  exhausted,  the  RAID0  driver  starts  collecting
>       chunks  into smaller stripes that only span the drives which still have
>       remaining space.
>
>   RAID1
> <snip>
>       All devices in a RAID1 array should be the same size.  If they are not,
>       then  only the amount of space available on the smallest device is used
>       (any extra space on other devices is wasted).
> <snip>
>
>   RAID4
> <snip>
>       Unlike RAID0, RAID4 also requires that all stripes span all drives,  so
>       extra space on devices that are larger than the smallest is wasted.
> <snip>
>
> My understanding is that this is why MD RAID0 does not yet support
> --grow, while MD RAID456 does - since they take different striping
> approaches, it's much harder to reuse the --grow algorithm.
>
>
> Good luck,
> Conway S. Smith
>
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Re: RAID0 on disks with different sizes

[Joachim Otahal]

My RAID0 answer was outdated. All space is used as described by 
"Beolach". In the example with 250/500/750 GB:
It would automatically stripe on all three for the for the first 250 GB, 
then switch to two drives on the 250GB to 500GB space, and to one drive 
for the rest.

I would play with it on files used via loopback first though.

Joachim Otahal

rj schrieb:
> Okay, not it is confusing.. having two different answers to this
> question :) Which answer is correct?
> RAID0 takes approach 1 or approach2?
>
> On Mon, Mar 15, 2010 at 1:28 PM, Beolach<beolach@gmail.com>  wrote:
>    
>> On Sun, Mar 14, 2010 at 23:26, rj<ratnadeep@gmail.com>  wrote:
>>      
>>> Hi,
>>>
>>> How does MD RAID0 layer support disks with different sizes?
>>> 1. Does it take the size of smallest disk and do the striping
>>> accordingly?  This way there would be wastage of space in bigger
>>> disks.
>>> Or
>>> 2. Does it stripe across all the space of disks? This way, if striping
>>> is not possible for a disk because it has exhausted the space,
>>> striping will continue or remaining devices (although with
>>> compromising performance for these stripes because they have one disk
>>> less).
>>>
>>> What approach md chooses? Also, what approach is taken for RAID5 in such cases?
>>>
>>>        
>> MD RAID0 follows #2; 4/5/6 follow #1.  From the man page for md:
>>
>>    RAID0
>> <snip>
>>        If devices in the array are not all the same size, then once the small‐
>>        est device has been  exhausted,  the  RAID0  driver  starts  collecting
>>        chunks  into smaller stripes that only span the drives which still have
>>        remaining space.
>>
>>    RAID1
>> <snip>
>>        All devices in a RAID1 array should be the same size.  If they are not,
>>        then  only the amount of space available on the smallest device is used
>>        (any extra space on other devices is wasted).
>> <snip>
>>
>>    RAID4
>> <snip>
>>        Unlike RAID0, RAID4 also requires that all stripes span all drives,  so
>>        extra space on devices that are larger than the smallest is wasted.
>> <snip>
>>
>> My understanding is that this is why MD RAID0 does not yet support
>> --grow, while MD RAID456 does - since they take different striping
>> approaches, it's much harder to reuse the --grow algorithm.
>>
>>
>> Good luck,
>> Conway S. Smith
>>
>>      

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