Re: Adding a smaller drive

[Bill Davidsen <davidsen@tmr.com>]

Leslie Rhorer wrote:
>>> 	I have a few questions.  Some RAID implementations will simply
>>> refuse to create or grow an array if all the targets are not precisely
>>>       
>> the
>>     
>>> same size.  Clearly this is not the case for mdadm.  Not all drives of a
>>> given "size" are actually precisely the same size, however, and I am
>>>       
>> using
>>     
>>> unpartitioned drives for my RAID systems.  What happens if I add a drive
>>> whose apparent physical size is a bit smaller than the device size used
>>>       
>> to
>>     
>>> create the array?
>>>       
>> For RAID 4/5/6, I think it'll be refused.
>>     
>
> 	Do you know if the refusal would include an error message clearly
> indicating why the growth is refused?
>
>   
>> You have to shrink the
>> filesystem, and LVM if you use it, then the array, so the used size is
>> no bigger than the new drive - as you've noted, md doesn't mind if it
>> doesn't use all the available space on its constituent devices. If it's
>> a small reduction, as I imagine it would be, and your filesystem
>> supports shrinking, it won't take long to do the the shrinks. Then
>> adding the new drive will be painless. If your filesystem won't shrink -
>> and some (many?) won't - I suspect you're scuppered.
>>     
>
> 	I'm no longer using LVM on any of the servers, and I've converted to
> XFS on RAID 5 and RAID 6 arrays.  At this time XFS does not support
> shrinking.  I've seen some chatter on the web about 3rd party utilities
> which might make it possible.
>
> 	For growing an array, this would be a bit of a pain, but probably
> not a show stopper.  Even for a failed drive I could probably just send the
> new drive back and purchase a different model whose real size is as large or
> larger than the extant drives.  The problem is, waiting that long for a new
> drive or doing anything significant (like multiple shrinks!) to a partially
> failed array sends shivers up my spine.
>
> 	I may have to rethink my position on using raw drives.  If I
> partition the drives, I can make the partition a bit smaller than the whole
> drive, allowing for the addition of a future drive whose size is a bit off.
> I hate to waste space, but being stuck with an undersized or limping array
> is worse.
>   

Some manufacturers use the HPA (host protected area) to reduce the size 
available to the user. You will see reference to this in the dmesg 
output. There is a tool to let you see/set HPA, but I can't put my hand 
on the info right now, the one I have is out of date, so I won't mention it.

-- 
Bill Davidsen <davidsen@tmr.com>
  Obscure bug of 2004: BASH BUFFER OVERFLOW - if bash is being run by a
normal user and is setuid root, with the "vi" line edit mode selected,
and the character set is "big5," an off-by-one error occurs during
wildcard (glob) expansion.