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On EC2, most of the larger current generation instances have multiple SSD devices. To take advantage of the combined size of these for applications like nosql databases that store large amounts of data, I've been combining them with software raid 0 or 10 using mdadm.

It takes forever when launching new instances though, for instance to run the initial raid10 sync on an i2.4xlarge instance takes several hours to complete.

Is there any way to speed this up? Is it possible to save an AMI after running the initial mdadm sync that will include all the ephemeral disks in the same raid configuration when it lauches?

I haven't used custom AMI's much but from reading the docs it sounds like both instance store backed and ebs backed AMI's only include a snapshot of the root volume.

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  • You don't want the ephemeral devices be part of your mdadm-managed array. Using ZFS -- with ephemeral device(s) being caches -- should work much better for you. See serverfault.com/questions/663774/…
    – Mikhail T.
    Mar 25, 2015 at 20:21
  • @MikhailT. thanks for the link, I'll definitely look into this configuration using ZFS if I run into this issue again
    – danny
    Mar 26, 2015 at 18:16
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    Don't think anyone will see this at this point, but can someone elaborate on why this was closed as "not relevant to professional systems administration"? It was an issue I ran into while being paid as a devops engineer/sysadmin, and the database I was setting up is a core part of system that now processes billions of dollars a year in payments. I asked a well-defined question and got a prompt answer that I accepted, why all the haters?
    – danny
    Mar 26, 2015 at 18:28
  • Is AWS itself off limits here?
    – danny
    Mar 26, 2015 at 18:32

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As you've noticed, ephemeral disks are not included in an AMI snapshot; only the root volume is. Because of this it's not possible to "snapshot" the metadata of mdadm because the ephemeral disks will be lost when the instance hosting it is destroyed. You can however create persistent RAIDs with EBS volumes.

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  • Thanks, good to confirm I wasn't just missing something.
    – danny
    May 12, 2014 at 15:15

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