Mongodb get Effective Lock

Your mongo stat shows higher number of updates vs inserts. One thing that could cause high write lock issues is if your updates typically are increasing the document size and causing the document to move in the data file. We ran into this ourselves, but we were working with mongo support at the time to figure out so I don't remember what metric or stat would tell you this is the case. This would likely only be an issue if your document sizes were very large. We ended up splitting out a sub array that was always being added to into its own collection so that we were just adding new documents instead of modifying an existing one.

The usePowerOf2Sizes flag on the collection can also help alleviate this by giving the documents room for growth. This is apparently the default now on 2.6, but you would need to turn it on if you're not on 2.6 yet. Setting that is described here: http://docs.mongodb.org/manual/reference/command/collMod/

MMS charts have multiple quantities you can add to determine the number of reads and the number of writes - at a glance they are:

• Reads:

  • queries
  • getmores

• Writes:

  • inserts
  • updates
  • deletes/removes

As a general ballpark value, the ratio of these is fine.

It's not quite this simple though because there are other read and write "loads" on the system:

• system writes which don't show up in counters, such as TTL deletes, etc.
• commands which may be reads: count, aggregate, in 2.4 and earlier getLastError().
• commands which may be writes: findAndModify, etc.
• on a primary a multi-update or remove that matches multiple documents is recorded as a single write operation (on secondary it would be a single write per document affected)
• a single read operation (aka find) may be a fast, indexed query or a slow collection scan of millions of documents.

Hope this helps, as in general you usually care about not exact absolute ratio of reads to writes, but rather how that ratio may be changing over time.