RAID 1 HDD degraded and want to know if it can be replaced with SSD

Posted on 2014-01-21
Last Modified: 2016-11-23
Have a Dell Server setup with Raid 1 and ONE of the HDD's degraded and am going to replace it. I wanted to know if I could replace it with the same size SSD without any problems. It is hardware raid!

The drives in the system is:
Seagate Barracuda ES.2 - Hard drive - 250 GB - SATA-300 - 7200 rpm - buffer: 32 MB - ST3250310NS

and want to replace the degraded one with
Samsung Electronics 840 EVO-Series 250GB 2.5-Inch SATA III  Solid State Drive MZ-7TE250KW

Could I run into any problems now or in the future if I did this?
Question by:georgopanos
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LVL 47

Expert Comment

ID: 39799121
It depends on the controller.  What do you have?
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Accepted Solution

David earned 334 total points
ID: 39799131
Actually, no it doesn't. The practical answer is no.  The reason is none of the LSI-manufactured controllers were tested in such a hybrid config.  Some controllers will flat-out prevent you from using a HDD and SSD in the same LUN. The controllers that won't prevent you haven't been certified.

The firmware won't be correct. This means huge potential for data loss in event of multiple I/O failure scenarios, and at the moment a HDD or SSD fails.

So don't do it if you value your data.
LVL 14

Expert Comment

ID: 39799165
You have to use exactly same drives in RAID, otherwise you run a very high chance of data corruption. It's not just about having the same capacity -- the drives should be identical so that any writes to both drives take the same amount of time and the data can be synced between them.

So, if you want to use SSD's, just take an image of the remaining dive (with something like Acronis Backup & Recovery), the get 2 new SSDs, andset up a new RAID1 array, and then restore that image onto the new array
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LVL 39

Expert Comment

by:Philip Elder
ID: 39799390
That's like powering one wheel with a v8 and the other with an L4 (4-cylinder).

Essentially the answer is no. Like with like is the rule of thumb. And, as mentioned the RAID controller very well may not allow an SSD and a spindle to be configured in the same array. Check the manufacturer's tested hardware list.

The other side of that is to use a pair SSDs that have been certified to work with the RAID controller in the server. Not doing so can also lead to data corruption or full array loss.

LVL 56

Assisted Solution

andyalder earned 166 total points
ID: 39799726
Drives don't have to be identical, you can run RAID 1 with a 10K and a 15K SAS for example, that mix can happen automatically if there's a 15K disk as a global spare. Writes don't take place at exactly the same time whatever drives you have since the spindles aren't synchronized to spin at exactly the same speed* and sectors on one disk do not exactly overlay those on another identical model due to spare sectors being used to cover for bad areas. This can lead to a write hole developing should power fail since the data on the disks is out of sync but you can mitigate against that by using a controller which has battery backed write cache.

Nevertheless I would concur that it's unlikely that a spinning disk can be mirrored to an SSD even if they're both SATA or both SAS.
LVL 47

Assisted Solution

David earned 334 total points
ID: 39799763
btdownloads7's comments about drives being the same are absolutely wrong.  As andy wrote, writes are asynchronous.  They won't start at the same time, they don't get done at the same time.  If you have a (write) caching controller then writes could happen well over a few seconds apart in extreme cases.   As such, it doesn't make sense that there could even be such a constraint.

As long as the HDDs have the appropriate firmware (and the controller firmware doesn't prevent it), then you can mix & match capacities, and performance characteristics.   You can even mirror a SSD with a HDD in some software RAID stacks such as ZFS, and will NOT get the lowest common denominator when it comes to performance.

Now some ancient BIOS-based fakeraid controllers do not do asnync writes, but we're talking 80386 type systems, and certainly not what you have.  So for all intents and purposes consider the myth that RAID requires the disks to be the same as busted.

P.S. Some LSI/Dell controllers absolutely prevent mixing / matching SSDs with non SSDs.   This is a OEM-configurable bit that gets set in the firmware.   I'm reasonably sure that Dell disables this for all of their controllers.   But it is moot, because the firmware of the SSD you have isn't qualified for mixing with a HDD anyway so you risk data loss even if that feature was enabled.
LVL 56

Expert Comment

ID: 39799790
There was a time when spindle synchronisation was used to eliminate the write hole but as dlethe says it's long ago, the Compaq SystemPro is the only machine I've ever come across that used it and that dates back to 1988.

Author Closing Comment

ID: 39799865
Excellent, Thank you!!!!

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