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