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I just read about Hybrid RAID arrays consisting of SSDs and common HDDs which are supported by Adaptec's RAID controllers.

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  1. Does such a RAID configuration provide real advantage?
  2. Isn't write performance downgraded to the writing speed of the HDD?
  3. What about different sizes of disks?

In short, what cons and pros has such a RAID solution?

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Went over the link you posted and it doesn't really make sense to me. If they write 50% of the data to the HDD, how can they avoid reading from it entirely? –  xbonez Feb 7 '11 at 21:27
    
@xbonez: I don't think they write 50% of data to HDD. This should be mirroring configuration where all data are written to both SSD and HDD (left image). –  Ladislav Mrnka Feb 7 '11 at 21:33
    
Oh, yes...sorry. I completely has RAID 0 in mind. –  xbonez Feb 7 '11 at 21:54
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3 Answers

up vote 6 down vote accepted

From the link:

Adaptec Hybrid RAID Solution Adaptec controllers write to both HDD and SSD and read from SSD 100% of the time resulting in maximum performance.

The writes and maximum volume size will be capped at the smallest/slowest disk like any other mixed RAID. The advantage is that reads are all from the SSD, which is where it really excels. It allows for the redundancy of RAID-1 and read-performance of a SSD without the cost of two SSDs.

Sure there is a write penalty, but for most home users they are read-heavy not write-heavy.

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Is RAID controller with support for SATA/SAS drives targeted to home users? –  Ladislav Mrnka Feb 7 '11 at 21:48
    
@Ladislav - Looking at the specs they are targeted at home power users or small office setups. A large enterprise probably wouldn't be buying a bare controller, and even if they did they probably wouldn't be mixing and matching drives. –  MDMarra Feb 7 '11 at 23:09
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Note that in addition to Adaptec's "Hybrid RAID", they also have something they call "Max Cache".

Hybrid RAID allows you to have your SSDs backed by regular spinning discs, to decrease the cost of the RAID array. The downside is that if you need the performance of the SSDs, you probably can't survive running on the mechanical disc. So you're getting the write performance of a spinning disc, and will probably have to take services down until the array rebuild completes -- seems pretty limited in usefulness to me. I'm speculating that you'd be better off with a 5 drive all SSD RAID-5 array than a 8 drive hybrid, but I don't have any numbers to back that up.

Max Cache allows you to put in an SSD as a cache device for your array of spinning discs. This sounds promising, but the fine print is that this cache is write-through only, so your writes happen at the speed of the spinning discs, and only reads are accelerated. If you need read acceleration, that's probably a pretty good way to go, but I haven't personally tested it.

If you are running 64-bit Linux, you can try the flashcache kernel module which allows you to do either write-through or write-back (though there may be data safety issues with write-back) caching of one block device to another. I've played with this and it works well, but in the situation I was planning to put it into production recently the system was 32-bit and it really does not work in 32-bit currently (July 2011).

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First of all, if you install SSD behind RAID controller you lose TRIM support.
Second, I don't really understand this whole idea of hybrid RAID. The whole point of RAID1 is IMHO being able to withstand loss of one of the drives. Now imagine you lose your SSD in this hybrid RAID under heavy load. You see my point? It looks more like a backup solution to me.

@MarkM Sorry for not expressing myself clearly. Of course controller will detect member fault. But why are people ready to pay premium for SSD? Because it can handle higher load. For some types of load (high IOPS) it can handle up to 10x compared with conventional HDD. Under such load the remaining HDD will never be able to work as SSD did - your server effectively goes down. So in case of RAID1 with 2 identical drives, when one goes down, server continues to operate - and that's the whole point of RAID1 IMHO. You have to replace the faulting drive not to restore the server (it's working okay) but because another drive might go down. But with this Hybrid RAID you have to replace faulty SSD as soon as possible to restore your server to normal operation. Like when you had only one drive and it failed and you have to replace it and restore from a backup. That's why I joked that it's a backup solution. Sorry again

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I'm sure that the controller is smart enough to read from the hard drive if the SSD drops from the array. Also, RAID is never a backup under any circumstance. –  MDMarra Feb 8 '11 at 3:04
    
I disagree. First, there are not only servers. Second, a server may get 10x slower after losing the disk, but it may be acceptable for a while. Third, losing one of two identical disks may reduce the read performance considerably. –  maaartinus Mar 15 '11 at 22:09
    
Just to add, there's a common real-world failure mode for Sandforce-based SSDs that causes them to die almost at random and without warning if your PC goes into suspend "the wrong way" due to a bug in the controller firmware. Making matters worse, if you try to RAID1 with a second identical SSD, the same bug can literally strike both drives simultaneously, leaving you equally screwed. In this case, being able to use the SSD as a write-through cache can mean the difference between spending a week or more recovering from a failure vs a few hours of aggravation reformatting the SSD. –  Bitbang3r Dec 13 '11 at 19:19
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