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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?
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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:

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+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 '13 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 Hybrid SSD/HDD RAID, 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.

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Thanks for the link. Also, "hybrid" is exactly the keyword whuch turns up more information in google - thanks a lot! –  oliver Jun 5 '11 at 12:49
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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 '11 at 13:22

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