Swapping out a raid array

slackcub · 2015-01-20 04:37:37

Hello:

I currently have a raid5 array for my desktop, but the disks are getting old and I'd like to replace it. Currently I'm using 3x500GB disks. I'd like to replace this with 4x2TB disks. In order to reduce the risk of data loss, I would rather add the new disks to the current array, then remove the old ones and resize the LVM volumes contained therein. Here are my questions:

1) Would it be possible to add the larger disks to the array and not be forced to use the currently smaller size?

2) What is the bigger risk to data? Adding the new disks to the current array and then removing the old ones, or simply building a new array and then copying the data over?

To top it all off, I only have 6 SATA ports currently and so i would have to do a resize either way once I start removing the old drives so I can add the last new one in. Any thoughts/tips/advice would be greatly appreciated!

ewaller · 2015-01-20 05:25:45

slackcub wrote:

I currently have a raid5 array for my desktop, but the disks are getting old and I'd like to replace it.

Are you planning on replacing the chassis as well (what are the drive housed in?)
Do you have a separate raid controller, or is this a software RAID?

Would it be possible to add the larger disks to the array and not be forced to use the currently smaller size?

From what I remember from my RAID 5 days, no. But, we used RAID 5 because we were doing 470 Mb/s television video back in the late 20th century and were using RAID to get the bandwidth. IIRC, replacement drives had to be the same geometry.

To top it all off, I only have 6 SATA ports currently and so i would have to do a resize either way once I start removing the old drives so I can add the last new one in.

I think that answered my previous question, you are using a software RAID.

headkase · 2015-01-20 05:31:55

This is related to RAID systems: http://www.pizzaandcode.com/posts/780

I'm not saying it will happen to you, I'm saying if you use RAID then it's worthwhile to read.

hydrosIII · 2015-01-20 08:47:03

Hi. from what you said, I assume you're using software RAID.

If you just build a new array, and copy the data over, it will require some downtime due to the file sync, but maybe it is the safest approach. Also maybe it is the simpler approach if your computer is a desktop computer and not a server, supposing that the computer doesn't need to be on the whole time, and that the data is not always changing. Depending of the amount of data you could do this in a night. ( Im guessing no more than 2Tb of data)

Other approach (more suited for servers) is just substitute one by one each of the old disks, waiting for the data to sync. When you got all your new disk in sync, then just upgrade from RAID 5 to RAID 6, and resize LVM.
This will take several days depending of the amount of data, but is painless and it has no downtime, however the computer has to be on during the whole process.

These article explains how to upgrade from RAID 5 to RAID 6.
http://www.ewams.net/?date=2013/05/02&v … 6_in_mdadm

slackcub · 2015-01-21 16:31:05

Yes, this is software RAID, and it's on my desktop. As much as I would like to avoid downtime of it, I'm thinking building the new array by itself and then copying the data over is the best way to go based on the comments here.

ewaller, they're housed inside my desktop, and I do have enough space for all the drives in there.

I will have to look into the benefits of RAID 6 over RAID 5 as I'm not really familiar with the former.

Thanks for the info everyone. It's given me a lot to think about!

alphaniner · 2015-01-21 16:59:07

The ideal solution is to get a SATA or SAS PCI card so all your drives are available at the same time.

Otherwise, I think the following could work:

1) Create a faulty degraded four drive RAID <whatever> on three of the new disks
2) Copy the data from the old RAID to the new
3) Safely remove the old RAID, such that it remains intact
4) Add the fourth new disk as a spare, which should prompt a rebuild

Barring issues with the SATA controller, this method pretty much involves no risk to your old RAID.

Regarding RAID levels: RAID 6 is just RAID 5 with a second set of parity data, meaning it can lose two drives instead of one. But the shortcomings of RAID 5 also apply to RAID 6. I suggest you go with RAID 10 rather than any parity scheme. Falconindy put it best:

If you care about data integrity, you use a mirror (more likely, RAID 10).

Last edited by alphaniner (2015-01-21 19:04:27)

slackcub · 2015-01-21 17:27:03

alphaniner, thanks for the input. While I am concerned about data integrity, I'm willing to take the risk with RAID 5/6. I'm just not willing to sacrifice that amount of storage lost to the mirroring.

alphaniner · 2015-01-21 18:56:53

FIY, with four drives RAID 6 involves the same level of sacrifice as RAID 10.

Honestly, I'm more interested to know if you end up going with the degraded RAID suggestion. FWIW it's not something I just pulled out of my arse. I don't remember where I read about it, but google came up with this which seems decent.

slackcub · 2015-01-21 19:01:05

alphaniner, that's probably why I'll stick with RAID 5. I'll have to look at that article.

Arch Linux

#1 2015-01-20 04:37:37

Swapping out a raid array

#2 2015-01-20 05:25:45

Re: Swapping out a raid array

#3 2015-01-20 05:31:55

Re: Swapping out a raid array

#4 2015-01-20 08:47:03

Re: Swapping out a raid array

#5 2015-01-21 16:31:05

Re: Swapping out a raid array

#6 2015-01-21 16:59:07

Re: Swapping out a raid array

#7 2015-01-21 17:27:03

Re: Swapping out a raid array

#8 2015-01-21 18:56:53

Re: Swapping out a raid array

#9 2015-01-21 19:01:05

Re: Swapping out a raid array

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