I recently switched a store cache and session backend to Redis 3.0.3. I used the Cm_Cache_Backend_Redis and Cm_RedisSession extensions and the example configs provided. It works as expected and Redis stores the sessions and cache in 2 separate DB's. However, the site gets intermittent Magento 503 service unavailable errors (as reported by customers and observed by loading several pages at once). If I disable the extensions and go back to the standard file session_save's the problem goes away and can no longer be reproduced.

Is there some tuning that needs to be done to Redis to handle higher volume? Has anyone else every experienced this?

  • Are there any entries in error.log or exception.log concerning this issue? What about Redis logs?
    – mkerstner
    Sep 1, 2015 at 5:07
  • Nothing, I have been monitoring all of the above.
    – mikeds
    Sep 1, 2015 at 6:00
  • 1
    What about apache error logs and/or php error logs? Maybe access logs or New Relic?
    – Erfan
    Sep 1, 2015 at 6:39
  • Sorry, but there must be some sort of error logging to be able to identify the problem at hand. And to answer your last question: No, we have so far not experienced problems with Redis handling higher volumes. Have a look at where Redis is currently logging in /etc/redis/redis.conf and check option "logfile".
    – mkerstner
    Sep 1, 2015 at 6:42

1 Answer 1


You're likely hitting max_concurrency. Colin Mollenhour has made significant updates to the Redis integrations (and his Redis Session integration) in his Github repo modules.

I suggest you upgrade to the most recent version, which batches SUNIONs to avoid massive wait times.

Aside from this I've seen max_concurrency hit while bots are crawling. Because of this we've, on occasion, made updates to the Bot settings in Collin's module.

See here for more information:


  • Thanks for the response. I bet max_concurrency was the issue. We were using the latest version of both of Colin's Redis modules. We have dropped Redis for now as it was causing an issue for customers on a live store. Hopefully I'll get a chance to revisit Redis and Colin's awesome extensions and play with max_concurrency a bit.
    – mikeds
    Sep 9, 2015 at 2:52

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