Does it make sense to build a Linux software RAID-1 (mirroring) using an SSD and a normal hard disk? I want to combine the advantages of SSD (fast transfer rates, fast access times) with the advantages of hard disks (cheap, different fault model) and get a very fast yet reliable RAID...

  • does this actually make sense?
  • is the SSD performance actually available in such a mixed setup? I.e. does Linux mainly use SSD for reading, or will it use the slow disk equally much?

2 Answers 2


Looks like this setup is quite ok. The basic rule is to use mdadm with --write-mostly parameter so that writes go to HDD. Reading will still be done from fast SSD. Also, the --write-behind parameter is said to improve write performance.

Support for TRIM command in such a setup appears to be a problem, though.

Further keywords for searching:

  • linux ssd raid hybrid
  • mdadm "--write-mostly" ssd

Further links:

  • 2
    +1. But remember that --write-behind is not safe, and because it needs a write-intent bitmap file external to the array it complicates setup and I wouldn't recommend it. --write-mostly is fine.
    – Tometzky
    Nov 9, 2013 at 11:47

The raid software will likely distribute the reads across the disks*. Given the read speed on the SSD is likely to be more than twice as fast as the slow disk, I would assume that you'd actually lose performance.

By your comment on fault model, I assume that you're concerned about the "wear out" rates of SSDs. Modern controllers use sophisticated wear levelling, so as long as you have a fair amount of free space on the drive mass amounts of writing isn't going to kill the drive any time soon.

*Edit: According to the answer at What advantages and disadvantages have Hybrid SSD/HDD RAIDs, it seems there are raid controllers/software that will intelligently bias the reads towards the SSD, so you won't lose performance by raiding it.

  • 1
    Thanks for the link. Also, "hybrid" is exactly the keyword whuch turns up more information in google - thanks a lot!
    – oliver
    Jun 5, 2011 at 12:49
  • 2
    Regarding wear leveling: It's true that it's unlikely to lose data on SSD due to "wear out" of flash blocks. However the controller and wear-leveling data itself is a single point of failure. Just assume your SSD controller board dies; you will likely be unable to recover any data even if it's physically still there. So SSD wear-leveling does not replace real mirroring.
    – SkyBeam
    Jun 5, 2011 at 13:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.