Re: status of raid 4/5 disk reduce

David Lethe wrote:

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-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of Michael Brancato
Sent: Wednesday, December 10, 2008 6:07 PM
To: Alex Lilley
Cc: linux-raid@vger.kernel.org
Subject: Re: status of raid 4/5 disk reduce


    

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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
    

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should be no issue with file system/lvm resizing as these can
      

generally
    

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grow on-line anyway.

I appreciate that shrinking the size of the array and doing so onto
      

less
    

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disks is both an unlikely requirement and fraught with danger.
      

Growing
    

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the size of the array but to less disks is very useful indeed, which
      

is
    

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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.

    

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This is a lot to ask for in terms of development, and creates
            

extreme
    

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risk of data loss.
First, you degrade /dev/md0, so any bad blocks or drive failures
            

will
    

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cause catastrophic
data loss, unless /dev/disk4 is used for mirroring in the
            

interim.
  

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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?

    

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Secondly, by removing that disk (for sake of argument, say each
            

disk is
    

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1TB. You go from 3TB usable data
to 2TB.  Most likely, you need to resize the file system in place
            

so it
    

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fits into 2TB.  You're probably booted
onto md0 also, which makes it difficult.  Resizing a hot
            

filesystem
    

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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
  

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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.
  

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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
  

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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
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Respectfully, go bother the LVM, jfs, ext, afs, and all the other file
system people.  You have zero chance of getting them on board to support
online file system shrinking without any guarantee of scratch space.
My advice is that you don't tell them you also want them to resize while
the md volume is being resized, and also don't tell them that the array
might be degraded.

If you want to copy 4x120 into 3x500 ... mount all the disks and COPY
the data.  If you are truly limited to 4 disks, and are too cheap to
spend $10-20 for another controller, after buying 1.5TB worth of disk
drives, then you really need to get your priorities in order.

David


  

The file system aspect is complete irrelevant - resizing is already possible in both directions.

Buying extra controllers isn't always appropriate.  there is the physical space issue in the case, power consumption of more drives not to mention heat and seeing as big drives get cheaper by the day why not get rid of all those small drives and replace with fewer larger drives.  It is completely reasonable and not a question of misplaced priorities, just doing the most sensible and most manageable thing.  less drives will always be better in raid 5 and 6 because the more drives you have the more chance there is of multiple failures.

Copying to a new array defeats the object of any reshaping and is completely impractical.

Rgds

Alex

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