Re: Some very basic questions

[Avi Kivity <avi@redhat.com>]

Ric Wheeler wrote:
>> You want to have spare capacity, enough for one or two (or fifteen) 
>> drives' worth of data.  When a drive goes bad, you rebuild into the 
>> spare capacity you have.
>
> That is a different model (and one that makes sense, we used that in 
> Centera for object level protection schemes). It is a nice model as 
> well, but not how most storage works today.

Well, btrfs is not about duplicating how most storage works today.  
Spare capacity has significant advantages over spare disks, such as 
being able to mix disk sizes, RAID levels, and better performance.

>>
>> When you replace the drive, the filesystem moves data into the new 
>> drive to take advantage of the new spindle.
>>
>
> When you buy a storage solution (hardware or software), the key here 
> is "utilized capacity." If you have an enclosure that can host say 
> 12-15 drives in a 2U enclosure, people normally leave one drive as 
> spare.  RAID6 is another way to do this. You can do a 4+2 and 4+2 with 
> 66% utilized capacity in RAID 6 or possibly a RAID5 scheme using like 
> 5+1 and 4+1 with one global spare (75% utilized capacity).
>
> That gives you the chance to do  rebuild your RAID group without 
> having to physically visit the data center. You can also do fancy 
> stuff with the spare (like migrate as many blocks as possible before 
> the RAID rebuild to that spare) which reduces your exposure to the 2nd 
> drive failure and speeds up your rebuild time.
>
> In the end, whether you use a block based RAID solution or an object 
> based solution, you just need to figure out how to balance your 
> utilized capacity against performance and data integrity needs.

In both models (spare disk and spare capacity) the storage utilization 
is the same, or nearly so.  But with spare capacity you get better 
performance since you have more spindles seeking for your data, and 
since less of the disk surface is occupied by data, making your seeks 
shorter.

-- 
error compiling committee.c: too many arguments to function