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



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.


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