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On Mon, Nov 28, 2011 at 8:10 PM, Michael Mol <mikemol@×××××.com> wrote: |
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> I've got four 750GB drives in addition to the installed system drive. |
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> |
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> I'd like to aggregate them and split them into a few volumes. My first |
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> inclination would be to raid them and drop lvm on top. I know lvm well |
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> enough, but I don't remember md that well. |
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> |
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> Since I don't recall md well, and this isn't urgent, I figure I can look at |
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> the options. |
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> |
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> The obvious ones appear tobe mdraid, dmraid and btrfs. I'm not sure I'm |
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> interested in btrfs until it's got a fsck that will repair errors, but I'm |
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> looking forward to it once it's ready. |
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> |
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> Any options I missed? What are the advantages and disadvantages? |
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> |
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> ZZ |
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|
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Hi Michael, |
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Welcome to the world of what ever sort of multi-disk environment |
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you choose. It's a HUGE topic and a conversation I look forward to |
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having as you dig through it. |
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|
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My main compute system here at home has six 500GB WD RE3 drives. |
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Five are in use with one as a cold spare. I'm using md. It's pretty |
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mature and you have good access to the main developer through the |
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email list. I don't know much about dm. If this is your first time |
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putting RAID on a box (it was for me) then I think md is a good |
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choice. On the other hand you're more system software savy than I am |
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so go with what you think is best for you. |
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|
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1) First lesson - not all hard drives make good RAID hard drives. I |
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started with six 1TB WD Green drives and found they made _terrible_ |
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RAID units so I took them out and bought _real_ RAID drives. They were |
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only half as large for the same price but they have worked perfectly |
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for nearly 2 years. |
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|
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2) Second lesson - prepare to build a few RAID configurations and |
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TEST, TEST, TEST __BEFORE__ (BEFORE!!!) you make _ANY_ decision about |
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what sort of RAID you really want. There are a LOT of parameter |
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choices that effect performance, reliability, capacity and I think to |
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some extent your ability to change RAID types later on. To name a few: |
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The obvious RAID type (0,1,2,3,4,5,6,10, etc.) but also chunk size, |
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metadata type, physical layout for certain RAID types, etc. I strongly |
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suggest building 5-10 different configurations and testing them with |
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bonnie++ to gauge speed. I didn't do enough of this before I built |
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this system and I've been dealing with the effects ever since. |
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|
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3) Third lesson - think deeply about what happens when 1 drive goes |
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bad and you are in the process of fixing the system. Do you have a |
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spare drive ready? Is it in the box? Hot or cold? What happens if a |
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second drive in the system fails while you're rebuilding the RAID? |
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It's from the same manufacturing lot so it probably suffers from the |
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same weaknesses. My decision for the most part was (for data or system |
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drives) 3-drive RAID1 or 5-drive RAID6. For backup I went with 5-drive |
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RAID5. It all makes me feel good, but it's too complicated. |
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|
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4) Lastly - as they say all the time on the mdadm list: RAID is not a backup. |
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|
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Personally I like your idea of one big RAID with lvm on top but I |
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haven't done it myself. I think it's what I would look at today if I |
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was starting from scratch, but I'm not sure. It would take some study. |
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|
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Hope this helps even a little, |
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Mark |