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I've got a hardware RAID1 of two Hitachi 15K drives here, forming the array with an Adaptec controller. Let's assume one disk dies and I'd like adopt SSDs. I would then replace the dead drive with a 90GB SSD, accepting that I waste ~20GB.

Is it okay to mix platters with a SSD in a hardware RAID1?

After the drives have re-synced, I could also replace the remaining 15K drive with SSD and theoretically the older SSD wouldn't have to "wait" for the disk anyore and the overall performance would explode to SSD specs.

That would be an elegant way to migrate my working system partition to SSDs. But would the HW RAID1 survive the temporary mixed hardware? Is the underlying hardware really transparent? Further, SSDs usually are SATA2, while my 15Ks are SAS, is that the catch?

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Depends entirely on the RAID controller, I personally don't see why this wouldn't work - assuming it is hot swappable, after all, an SSD can be seen as a regular hard drive - that being said, I wouldn't recommend it, and unless the controller has the ability to grow - you will always loose that 20GB and more importantly, unless your controller was built for SSDs, you will not be able to perform TRIM or other SSD features - so, in the long run, performance and life may be significantly decreased.

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Controller is a ~2008 IBM ServeRAID 8k, relabeled Adaptec 3805 8P I think, and as you say, no TRIM, not designed for SSDs. Does that change anything of your answer? – isync Jan 24 '12 at 22:01
If it isn't designed for SSD, it will work, I just wouldn't recommend it... That being said, if your SSD has a good warranty and you are happy with performance loss over time (not drastic and it will still be faster than your SAS (I think)), then, why not! – William Hilsum Jan 24 '12 at 22:06
I agree. It should work, it's just a bad idea. – Shinrai Jan 24 '12 at 22:42
I wouldn't necessarily say it is a bad idea. Sometimes the cost/benefits of the situation warrants the performance. – surfasb Jan 25 '12 at 8:33

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