Re: status of raid 4/5 disk reduce

[Michael Brancato <mike@mikebrancato.com>]

> There is the very obvious use to reduce the number of drives but
> ultimately have a larger array if the drives are all larger. And there
> should be no issue with file system/lvm resizing as these can generally
> grow on-line anyway.
> 
> I appreciate that shrinking the size of the array and doing so onto less
> disks is both an unlikely requirement and fraught with danger.   Growing
> the size of the array but to less disks is very useful indeed, which is
> what I was getting at.

Hardware limitations is a good use case.  When I say reduce, I mean 
--grow -nX and not necessarily reducing the size of the array in the end.

>>>> This is a lot to ask for in terms of development, and creates extreme
>>>> risk of data loss.
>>>> First, you degrade /dev/md0, so any bad blocks or drive failures will
>>>> cause catastrophic
>>>> data loss, unless /dev/disk4 is used for mirroring in the interim.

This is a standard fact of RAID45.  Any RAID45 with a failed drive is 
subject to these same concerns.  Isn't this true today with grow if 
replacing a 4x100GB array with 4x200GB by replacing one drive at a time?

>>>> Secondly, by removing that disk (for sake of argument, say each disk is
>>>> 1TB. You go from 3TB usable data
>>>> to 2TB.  Most likely, you need to resize the file system in place so it
>>>> fits into 2TB.  You're probably booted
>>>> onto md0 also, which makes it difficult.  Resizing a hot filesystem
>>>> without scratch space??  If your file system
>>>> can't be dynamically reduced, then no point worrying about md raid.

There are a lot of assumptions here about how the array is used, 
filesystem support, etc.  I'm not saying that in every situation this is 
ideal.  There are many situations where md0 is not the boot device, md0 
is not the device to be contracted, and the filesystem supports either 
online or offline resizing.  Concerns about filesystem expansion or 
contraction (online or not) and array shrinking are mutually exclusive 
of one another and shrinking the size of the array is already possible.

Neil Brown has previously responded to a comment on the topic at 
http://neil.brown.name/blog/20050727143147 in regards to a --shrink option.

Here are a few use cases:

Hardware limitations - Replacing 4x120GB size drives with 3x500GB 
drives.  This would involve replacing each 120GB disk with a 500GB one 
at a time and rebuilding each before reshaping the array to 3 drives and 
growing to use all space on the new drives.  This is especially useful 
on a system which cannot increase the number if drives it has (4 max), 
only capacity.

Drive failure - A developer, home user or SMB has a drive failure in an 
array.  Due to money, time, shipping delays, etc, the user cannot 
replace the drive immediately and the drive is in a degraded state.  The 
user shrinks the filesystem by 1 drive amount and shrinks the array to 
return to a optimal state in the array.  The array would return to a 
protected state in hours not days if waiting on a drive.

Flexibility - A user wishes to free a disk in an array which is 
oversized to use that disk elsewhere.

I hope this give a better understanding of the usefulness of reducing 
the amount of disks in a RAID45 array.
--
Mike Brancato, CISSP