Planning for RAID failure: Can / should the drives' disk space not be used completely?

[etoh]

Hello,

I want to setup a RAID system (RAID-6 with 6 HDs, each having 4 TB capacity, using an Adaptec RAID 6805 controller). I read several articles about the subject, but I need help with the following question: Planning for drive failure, I wonder if it's possible and / or advisable not to use the drives' full capacity. The reason why I ask is this: Once I have to replace one of the HDs, I assume that I might have to buy different HDs than I will buy for the initial setup. While I would try to buy drives with (at least) the same capacity, I'm afraid that the actual capacity of the new drive might just be a few KBs less than the capacity of the old drives, making the new drive unsuitable for a rebuild of the RAID (or does this problem not even exist?).

To be a little more specific, I'm wondering...

• 
  ■...if it is advisable not to use the entire available disk space for setting up the RAID, but let's say a few MBs less, assuming that this is even possible.
  ■...if it's not possible or feasible to use less than the entire disk space, what would you suggest to prevent this from becoming a problem?

Thank you!

 

[USAFRet]

It is not advisable to use the whole space in ANY drive config.
Single, RAID, whatever.

It should never ever be 100% full. So a few kb difference does not matter.


For instance, I just replaced the 4 x 3TB drives in my RAID 5 NAS with 4 x 4TB.
7.5TB effective to 10.6TB effective.
Replacing them one by one....the total array size did not adjust until the 4th one was in place.
The RAID array will adjust to meet the size of the smallest. And if only a few kb diff, no problem.

 

[etoh]

Thank you for answering!

Yet, I'm afraid there was a misunderstanding: When I wrote about not using the disks' space completely, I wasn't referring to the space of the resulting logical volume, which will be roughly 16 TB. I knew that I should not use the entire 16 TB to save files. What I was trying to ask was whether I should somehow find a way to tell my RAID controller not to use the entire 4 TB of each drive for the RAID, but e.g. just 3.99 TB.

One related problem is: In the Installation And User's Guide of my RAID controller, I'm unable to find any way to not use the disk space in its entirety for creating a RAID. If the nomenclature for this option is not self-explanatory for RAID newbies like me, could someone please tell me a keyword that will help me to look it up?

In addition, on page 74 in chapter 11.4.2 it says

Make sure that the new disk drive is equal or greater in size than the failed disk drive.

Therefore: Are you absolutely certain that one can use a smaller drive when it's just about a few KBs?

 

[USAFRet]

They're talking about "same size" in the GB or TB magnitude. Using drives from different manufacturers in a RAID array is very common, often recommended.
This is to ward off the possible Common Mode Failure situation.

If you buy drives of the same type and model, at the same time...if one fails, the others are 'more' likely to fail from the same thing than if you had 2 or 3 different 4TB drives. Seagate+WD+HGST.

I have 4 x Seagate Ironwolf drives. If one fails, I'd have zero concern in replacing it with a 4TB WD Red.

 

[etoh]

Thanks again! Both of your replies taken together answer my question.