mdadm, raid1: write to multiple devs but read (mostly) from one?

Question

Is it possible to prioritize which devices are for writes/reads in mdadm RAID1 setup? I'd like to write to both devices for fault tolerance (AWS EBS and local storage SSD), but read mostly from local SSDs to improve latency and avoid excessive AWS I/O fees?

Above setup is for MySQL data. Ideally, queries would be handled by the local SSD serving as cache, but writes would go to both EBS and SSD devices.

Follow up: I found this article http://tansi.info/hybrid/ Does anybody see a problem if HDD was replaced by EBS in above setup?

Answer: the exact setup is described here http://hatim.eu/2014/05/25/leveraging-ssd-ephemeral-disks-in-ec2-part-2/

Answer 1

You can use -W option when creating the array. From the manual:

-W, --write-mostly
              subsequent devices listed in a --build, --create, or --add  com‐
              mand will be flagged as 'write-mostly'.  This is valid for RAID1
              only and means that the 'md'  driver  will  avoid  reading  from
              these devices if at all possible.  This can be useful if mirror‐
              ing over a slow link.

To further optimize your setup, give a look to --write-behind also.