On Tue, May 19, 2009 at 9:09 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Sat, May 2, 2009 at 2:46 PM, raz ben yehuda <raziebe@xxxxxxx> wrote:
>> Neil Hello
>> The bellow is the raid0 grow code.I have decided to fix raid0 and not
>> perform the transformation raid0-raid4-raid0 due to two reasons:
>> 1. raid0 zones. this patch support any zone transformations.
>> 2. Undesired dependency of raid0 over raid4 re-striping code.
>
> Hi Raz,
>
> Can you explain a bit more about why the raid4 approach is
> undesirable? I think making reshape only available to raid0 arrays
> where all the members are the same size is a reasonable constraint.
1. Because this is the general case. what will you tell a user with two zones ?
2. In my next generation systems, raid0 is on top of 12 raid5s.
A user may choose to expand one of these raid5s separately.
3. The $/Giga ratio will not decline while it should.
Storage lasts years, disks change sizes and formation. Segate
just announced a 2TB disk.
If you start with 4 1TB disks, and in for 4 years you expand this
array 4 times, each time
with a 2TB disk, you end up with 8 TB out of 12 TB. You lose
30% storage. Tell that to your
marketing and tell me when you do that, I want to see :)
4. Number of raid components .
To reach Size X , you need more components with equal size.Having
multiple zones lets you increase
the raid size without being subordinate to an ever increasing
number of disks. you just mirror one of the disks to a bigger disk,
put the new disk back in the array, and reshape with the extra space
of the new disk.
Note !
I do not know what is your code base, but if you still have the hash
spacing, you might
have two zones even if the underlying components are the same in size.
So, in this case your decision if to support or not support the raid0
reshape is not just nb_zone > 1.
Other than that, if you do this work, it be will nice to have my back
covered sooner than expected.(as long as you will NOT reach for your
revolver.... :) )
> We then get the nice benefit of reusing the raid5 reshape
> infrastructure. In other words I am not convinced that the benefits
> of reimplementing reshape in raid0 outweigh the costs.
> Thanks,
> Dan
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