software MD raid - 1 big array or multiple arrays?

R

Red Squirrel

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I know with SANs it's best to have multiple smaller luns instead of one big lun, but wondering if this is different when it comes to software raid. Debating on if I should make another raid 5 array by filling the 4 empty drive slots, or if I should just keep growing my existing array. I have 7 slots total to play with (3 in use now). So should I make one big array with the 7 slots, or should I make 2 separate arrays?

Also if I throw in a 2TB drive I should be able to add it in with the other 1TB drives right? It will just cap at 1TB? I figure if I'm going to buy drives, may as well go 2TB from now on, and eventually phase out the 1TB ones.
 
I think it really depends on your needs, if you have good backups, and how much the data means to you. As the number of drives goes up, so do your chances of having a drive failure; this in turn raises your chances of losing the array. You can push this off a bit by going with RAID-6 (which allows for 2 drive failures), but if possible, you'll still want a backup of some kind.
 
7 drives isn't a number you generally have to split up unless you have very specific requirements.
 
I have good backups, so other then the PITA it would be to reorganize everything, it's not lost. I don't backup the VM files themselves but I backup those as if they were machines. So I'd still have to reinstall the OS and what not.

Also should I see a boost in performance or will it be the same? From my understanding raid5 performance increases with more drives.

And yeah think if ever I end up with 7 drives, I will either go raid6, or have a spare drive onhand vs having to order another and waiting while it's in degraded mode.
 
From what I have read, here's what I'd do:

Buy three 2TB drives and create a RAID 5 from them. Then copy the data from the 1TB drives to the 2TB drives. You're now free to re-purpose/sell the 1TB drives, and if you need to grow your array, simply buy 2TB units and expand the array.
 
More spindles will provide more performance, barring any other bottlenecks.

I used HW RAID6 on one of my servers for a while (7 disk). Had a drive die out of the blue, and while I had a full backup of the data, it ran just fine until I got the RMA drive back (~5 days). Once you start using more, larger drives, rebuilt times increase, which increase your timeframe of chancing another failure while rebuilding. While I wont' say that its required for home use, if you have the ability, RAID6 provides more safety before you have to resort to backups/rebuilds.

There are a few really good papers on chances of failures of different array types.
 
Yeah the more I think of it, I think raid6 is something I want to look into for sure. Even if I do have a spare onhand or even a hot spare, I still end up with a period of a day or so where if another drive fails, I'm pooched. If I recall the rebuild takes a good 8 hours now, so double that with 2TB drives.

Eventually when I fill all bays with 2TB drives, that's still 10TB of highly redundant disk space. :eek:

I'm also debating on sticking with 1TB drives for now due to the price difference. By the time I need anything more then 5TB I will probably be looking at building a new server anyway. Those norco cases look real nice, I want to build a san some time when I have more money.
 
Probably, if I wanted to I could just buy two 1TB drives and I can probably tell it to convert to raid6 too. I have to read up on it though, but I'm pretty sure it's possible. MD raid is quite amazing. I will probably only go raid 6 when I add the last two drives. I'll stick to raid 5 for now as my odds are still low enough with 4 drives.

I've been hovering at around 100GB of free space for the past few months, so I'm cutting it close lol. There's some stuff I can probably cleanup though. Got lot of old acronis "before" images of people's PCs I worked on. They have not called me to say stuff is missing, so I can probably delete those.

Edit: confirmed: http://neil.brown.name/blog/20090817000931
 
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