In Parts 1 and 2 we have covered a number of common issues people run into when managing a sharded MongoDB cluster. In this final post of the series we will cover a subtle, but important distinction in terms of balancing a sharded cluster as well as an interesting limitation that can be worked around relatively easily, but is nonetheless surprising when it comes up.

6. Chunk balancing != data balancing != traffic balancing

The balancer in a sharded cluster cares about just one thing:

Are chunks for a given collection evenly balanced across all shards?

If they are not, then it will take steps to rectify that imbalance. This all sounds perfectly logical, and even with extra complexity like tagging involved the logic is pretty straight forward. If we assume that all chunks are equal, then we can rest assured that our data is being evenly balanced across all the shards in our cluster and rest easy at night.

Although that is sometimes, perhaps even frequently, the case it is not always true - chunks are not always equal. There can be massive “jumbo” chunks that exceed the maximum chunk size (64MiB), completely empty chunks and everything in between.

Let’s use an example from our first pitfall, the monotonically increasing shard key. For our example, we have picked just such a key to shard on (date), and up until this point we have had just one shard and had not sharded the collection. We are about to add a second shard to our cluster and so we enable sharding on the collection and do the necessary admin work to add the new shard into the cluster.

Once the collection is enabled for sharding, the first shard contains all the newly minted chunks. Let’s represent them in a simplified table of 10 chunks. This is not representative of a real data set, but it will do for illustrative purposes:

Table 1 - Initial Chunk Layout

Now we add our second shard. The balancer will kick in and attempt to distribute the chunks evenly. It will do this by moving the lowest range chunks to the new shard until the counts are identical. Once it is finished balancing, our table now looks like this:

Table 2 - Balanced Chunk Layout

That looks pretty good at the moment, but lets imagine that more recent chunks are more likely to have more activity (updates say) than older chunks. Adding the traffic share estimates for each chunk shows that shard1 is taking far more traffic (72%) than shard2 (28%) despite the chunks seeming balanced overall based on the approximate size. Hence, chunk balancing is not equal to traffic balancing.

Using that same example, let’s add another wrinkle - periodic deletion of old data. Every 3 months we run a job to delete any data older than 12 months. Let’s look at the impact of that on our table after we run it for the first time (assuming the first run happens on July 1st 2015).

Table 3 - Post-Delete Chunk Layout

The distribution of data is now completely skewed toward shard1 - shard2 is in fact empty! However, the balancer is completely unaware of this imbalance - the chunk count has remained the same the entire time, and as far as it is concerned the system is in a steady state. With no data on shard2, our traffic imbalance as seen above will be even worse, and we have essentially negated the benefit of having a second shard for this collection.

Possible Mitigation Strategies

If data and traffic balance are important, select an appropriate shard key

Move chunks manually to address the imbalances - swap “hot” chunks for “cool” chunks, empty chunks for larger chunks

7. Waiting too long to shard a collection (collection too large)

This is not very common, but when it falls on your shoulders, it can be quite challenging to solve. There is a maximum data size for a collection when when it is initially split which is a function of the chunk size and data size as noted on the limits page.

If your collection contains less than 256GiB of data, then there will be no issue. If the collection size exceeds 256GiB but is less than 400GiB, then MongoDB may be able to do an initial split without any special measures being taken.

Otherwise, with larger initial data sizes and the default settings, the initial split will fail. It is worth noting that once split the collection may grow as needed and without any real limitations as long as you can continue to add shards as data size grows.

Possible Mitigation Strategies

Since the limit is dictated by the chunk size and the data size, and assuming there is not much to be done about the data size, then the remaining variable is the chunk size. This is adjustable (default is 64MiB) and can be raised in order to let a large collection split initially and then reduced once that has been completed.

The required chunk size increase will depend on the actual data size. However, this is relatively easy to work out - simply divide your data size by 256GB and then multiply that figure by 64MiB (and round up if it is not a nice even number). As an example, let’s consider a 4TiB collection:

4TiB divided by 256GiB = 16 64MiB x 16 = 1024MiB

Hence, set the max chunk size to 1024MiB, then perform the initial sharding of the collection, and then finally reduce the chunk size back to 64MiB using the same procedure. .

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