Re: [linux-lvm] Building up a RAID5 LVM home server (long) [0x05B52F13]

[Sam Vilain <sam@vilain.net>]

Andy Boyko wrote:
>>The underlying principle, is keeping the MD layer as simple as possible,
>>so that you've got nice little resilient building blocks for your Volume
>>Group.  Never use concatenation, striping or RAID5 at the MD layer if
>>you've got LVM, it's simply in the wrong place!
> Could you by any chance explain this?  Assuming, say, five identical
> disks, is it really wrong to make an MD RAID-5 out of them, and then
> put an LVM LV atop that, for later extension?   I'm feeling pretty 
> dopey, 'cause I'm not immediately grasping what would be an alternative 
> MD/LVM/EVMS/whatever approach, assuming you do fundamentally want N-1 
> worth of available space from those disks.  What am I missing?

Sorry, I shouldn't have said "never".  All I'm saying is that most of
the time, you think you want this but in balance you probably want to
simply buy a few more disks, or bigger disks, instead of worrying
about an extra X% of space, which invariably will just increase
backup requirements anyway.

If you do have an equally sized (ideally also homogenous brand/model)
set of disks, *and you don't overly care about write performance or
excessive performance degradation in the case of a single failure*,
then sure, RAID 5 setup as you say is the right solution.

This is just an old saying repeated.

   RAID - Fast, Cheap, Reliable.  Pick any two.

fast+cheap = raid 0/striping
fast+reliable = raid 1 or 10
cheap+reliable = raid 5

Sam.