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unseengundam 01-31-2013 10:52 AM

Questions on Linux Raid (mdadm) and how well it works?

I have been thinking about creating a RAID using Linux Fake raid or mdadm tool instead of just keeping on adding single HD to my system. However, I have couple question on mdadm raid:

1.) If my Motherboard / SATA controller fails, can I transfer/recover my RAID and data if I transfer to new computer or MB? I have had this scenarios before motherboard failed but luckily since they were just single drives I could just plug into a new motherboard.

2.) How long does it take to Grow/Add to a Raid 5 array? Ex: I add 2 TB drive to a 4x2TB array. I have heard Raid 5 especially takes a long time.

3.) Will the data on Raid 5 be access able during rebuild/recover/adding task on mdadm?

4.) Finally, I keep on hearing Raid 10 is better option than Raid 5, any opinion on this as related to Linux fake software raid?


gtrawoger 01-31-2013 01:44 PM

Hi unseengundam,

I can tell you want I used my RAID for, but there are much wiser people on this forum that may correct me on a couple points.

First of all, I would recommend mdadm over fakeraid, since fakeraid will lock you to the chipset you were using. So if your motherboard needs to be replaced, you will most likely have to replace it with the same one.

mdadm on the other hand can be put on any motherboard, and then exchanged. I actually just did that when I went to replace an ASUS board with a Supermicro one (finally had the money to put something proper in place).

I am not sure why you would need RAID 10 over RAID 5, since all it would improve is the write speeds, but you lose 1/2 space of all drives, while in RAID 5 you only lose 1 drive's space.

So if you are after performance+reliability, then RAID 10. If you are after space+reliability, then RAID 5 (or 6).

I have a 10TB (8x 2TB HDD - 7 Active & 1 Hot Spare) RAID 6 setup. RAID 6 can survive 2 disk failures vs. the 1 disk failure of RAID 5.

I also have a 4TB RAID 5, and a 2TB RAID 5, all working just fine.

A complete resync will take around 300 minutes (if you add a new drive) if you adjust the default parameters. With default it will take 5x as long.

And yes, the array is accessible during rebuild/recover/add, but it will be slower. The only times it will not be accessible if the array cannot be started (too many drives failed/disconnected).

I would look up some test results as to what to make your stripe size and chunk size (filesystem). These can make the biggest difference in performance from what I could see.

I have 4K stripes and 4K chunks, get almost 1GB read and ~600MB write speeds on my array. I am sure that could be tweaked further, but I didn't have the time.

Hope that helped.

unseengundam 01-31-2013 05:55 PM

Hey gtrawoger,

That actually really good info there! My goal was to have more space and some reliability. For example, I could do Raid 0 or JBOD setup but increase risk of failure over single drive. This primary why I was considering Raid 5.

The motherboard swap you accomplished is just type of task I wanted get info on! I noticed motherboard can fail and so do Raid controller cards. Being able to recover the data with having get exact same motherboard, controller/chipset is a great plus for mdadm.

Thanks for the info!

gtrawoger 02-01-2013 10:46 AM

You're welcome.

Don't be afraid to play around with the RAID array, like what happens when you unplug a drive, what happens when you change the stripe size, etc.
Just don't use the whole drive, maybe just the first 50GB or so, so that the re-sync and build won't take forever.
That way you learn what to do when things go wrong. Also, setup the email notification that mdadm has build in. It's a life saver when you don't check the array every day.

Once you played with it for some time, then set it up and put your data on it.

jefro 02-01-2013 03:47 PM

" Being able to recover the data with having get exact same motherboard, controller/chipset is a great plus for mdadm."

I think that is worded wrong as typo. It is without having to have the exact same board.

A long time ago, software based raid didn't really perform any better than a single drive. Of course mirror helped for data safety. For speed, the only way to improve it was with hardware raid. I wish some web site had some current data on whether or not any speed improvement was to be had with software raid on today's systems.

As above, I'd stay away from any fake raid solution even if it seems to work.

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