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I have been tasked with adding some large disks to a server that is several years old. The raid card in this server can support 12 disks, but its age prevents it from achieving any raid disk > 3TB.

I know a lot about RAIDs but not a lot about LVM.

In the end, I have setup two RAID5 arrays of 3TB each. On the server, it would be nice to use LVM to combine them into one disk. Two questions:

  1. Am I gaining anything at all by using RAID in the first place if I'm just going to combine it all in the end using LVM?

  2. Am I losing the security of RAID5 by combining the two RAID5 disks into one disk using LVM? Does it defeat the purpose of using RAID5 to begin with.

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What OS/distro? How about software RAID 0 across 6x RAID 1 arrays or 4x RAID 5 arrays? –  Andrew Apr 5 '11 at 3:12

2 Answers 2

I think you are doing it right. Purpose of RAID to provide different degrees of redundancy and performance. The purpose of volume manager is to build volumes from reliable "chunks" of storage provided by RAID. The functionality overlaps in some cases, but I like to keep it separate. So, I would do the exactly same thing in your situation.

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As long as both RAID groups stay healthy, there should be no issue.

Think of your new setup as RAID 5+0 (minus the performance benefits of the 0); you can lose 1 disk per group without issue.

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Actually, you can have the performance benefits of the 0 using LVM striping. In this case, lvcreate --size $SIZE --stripes 2 --stripesize $RAID_STRIPE_SIZE --name $NAME $VG $PV_1 $PV_2 –  sciurus Apr 5 '11 at 4:21
    
@sciurus Good point, thanks! –  Shane Madden Apr 5 '11 at 4:22
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@sciurus, There are several reasons I would recommend against it. You will lose flexibility of adding chunks of different sizes. Plus you are truing to do RAID controller job with LVM (i.e. resemblance of software RAID on top of hardware RAID). Another reason is that when I lay out a file system I align it to RAID dimensions, i.e. stripe size/stripe width, however if you apply striping on top of it it screws up this alignment. So, I would say NO to stripping in LVM. –  dtoubelis Apr 5 '11 at 6:54
    
But mirroring might be worth considering if latency is an issue –  symcbean Apr 5 '11 at 8:40
    
@symcbean, I'm talking about simple concatenation. I'm not sure if LVM can even do mirroring, but if it does - I would recommend against it either. Why waste disk space. –  dtoubelis Apr 5 '11 at 14:49

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