# 5.2. Performance

With up to tens of thousands of documents you will generally find CouchDB to perform well no matter how you write your code. Once you start getting into the millions of documents you need to be a lot more careful.

## 5.2.1. Disk I/O

### File Size

The smaller your file size, the less I/O operations there will be, the more of the file can be cached by CouchDB and the operating system, the quicker it is to replicate, backup etc. Consequently you should carefully examine the data you are storing. For example it would be silly to use keys that are hundreds of characters long, but your program would be hard to maintain if you only used single character keys. Carefully consider data that is duplicated by putting it in views.

### Disk and File System Performance

Using faster disks, striped RAID arrays and modern file systems can all speed up your CouchDB deployment. However, there is one option that can increase the responsiveness of your CouchDB server when disk performance is a bottleneck. From the Erlang documentation for the file module:

> On operating systems with thread support, it is possible to let file operations be performed in threads of their own, allowing other Erlang processes to continue executing in parallel with the file operations. See the command line flag +A in erl(1).

Setting this argument to a number greater than zero can keep your CouchDB installation responsive even during periods of heavy disk utilization. The easiest way to set this option is through the `ERL_FLAGS` environment variable. For example, to give Erlang four threads with which to perform I/O operations add the following to `(prefix)/etc/defaults/couchdb` (or equivalent):

    export ERL_FLAGS="+A 4"

## 5.2.2. System Resource Limits

One of the problems that administrators run into as their deployments become large are resource limits imposed by the system and by the application configuration. Raising these limits can allow your deployment to grow beyond what the default configuration will support.

### CouchDB Configuration Options

#### delayed_commits

The `delayed commits` allows to achieve better write performance for some workloads while sacrificing a small amount of durability. The setting causes CouchDB to wait up to a full second before committing new data after an update. If the server crashes before the header is written then any writes since the last commit are lost. Keep this option enabled on your own risk.

#### max_dbs_open

In your configuration (local.ini or similar) familiarize yourself with the `couchdb/max_dbs_open`:

    [couchdb]
    max_dbs_open = 100

This option places an upper bound on the number of databases that can be open at one time. CouchDB reference counts database accesses internally and will close idle databases when it must. Sometimes it is necessary to keep more than the default open at once, such as in deployments where many databases will be continuously replicating.

### Erlang

Even if you’ve increased the maximum connections CouchDB will allow, the Erlang runtime system will not allow more than 1024 connections by default. Adding the following directive to `(prefix)/etc/default/couchdb` (or equivalent) will increase this limit (in this case to 4096):

    export ERL_MAX_PORTS=4096

CouchDB versions up to 1.1.x also create Erlang Term Storage (ETS) tables for each replication. If you are using a version of CouchDB older than 1.2 and must support many replications, also set the `ERL_MAX_ETS_TABLES` variable. The default is approximately 1400 tables.

Note that on Mac OS X, Erlang will not actually increase the file descriptor limit past 1024 (i.e. the system header–defined value of `FD_SETSIZE`). See this tip for a possible workaround and this thread for a deeper explanation.

### Maximum open file descriptors (ulimit)

Most *nix operating systems impose various resource limits on every process. The method of increasing these limits varies, depending on your init system and particular OS release. The default value for many OSes is 1024 or 4096. On a system with many databases or many views, CouchDB can very rapidly hit this limit.

If your system is set up to use the Pluggable Authentication Modules (PAM) system (as is the case with nearly all modern Linuxes), increasing this limit is straightforward. For example, creating a file named `/etc/security/limits.d/100-couchdb.conf` with the following contents will ensure that CouchDB can open up to 10000 file descriptors at once:

    #<domain>    <type>    <item>    <value>
    couchdb      hard      nofile    10000
    couchdb      soft      nofile    10000

If you are using our Debian/Ubuntu sysvinit script (`/etc/init.d/couchdb`), you also need to raise the limits for the root user:

    #<domain>    <type>    <item>    <value>
    root         hard      nofile    10000
    root         soft      nofile    10000

You may also have to edit the `/etc/pam.d/common-session` and `/etc/pam.d/common-session-noninteractive` files to add the line:

    session required pam_limits.so

if it is not already present.

For systemd-based Linuxes (such as CentOS/RHEL 7, Ubuntu 16.04+, Debian 8 or newer), assuming you are launching CouchDB from systemd, you must also override the upper limit by creating the file `/etc/systemd/system/<servicename>.d/override.conf` with the following content:

    [Service]
    LimitNOFILE=#######

and replacing the `#######` with the upper limit of file descriptors CouchDB is allowed to hold open at once.

If your system does not use PAM, a ulimit command is usually available for use in a custom script to launch CouchDB with increased resource limits. Typical syntax would be something like ulimit -n 10000.

In general, modern UNIX-like systems can handle very large numbers of file handles per process (e.g. 100000) without problem. Don’t be afraid to increase this limit on your system.

## 5.2.3. Network

There is latency overhead making and receiving each request/response. In general you should do your requests in batches. Most APIs have some mechanism to do batches, usually by supplying lists of documents or keys in the request body. Be careful what size you pick for the batches. The larger batch requires more time your client has to spend encoding the items into JSON and more time is spent decoding that number of responses. Do some benchmarking with your own configuration and typical data to find the sweet spot. It is likely to be between one and ten thousand documents.

If you have a fast I/O system then you can also use concurrency - have multiple requests/responses at the same time. This mitigates the latency involved in assembling JSON, doing the networking and decoding JSON.

As of CouchDB 1.1.0, users often report lower write performance of documents compared to older releases. The main reason is that this release ships with the more recent version of the HTTP server library MochiWeb, which by default sets the TCP socket option SO_NODELAY to false. This means that small data sent to the TCP socket, like the reply to a document write request (or reading a very small document), will not be sent immediately to the network - TCP will buffer it for a while hoping that it will be asked to send more data through the same socket and then send all the data at once for increased performance. This TCP buffering behaviour can be disabled via `httpd/socket_options`:

    [httpd]
    socket_options = [{nodelay, true}]

See also

Bulk load and store API.

### Connection limit

MochiWeb handles CouchDB requests. The default maximum number of connections is 2048. To change this limit, use the server_options configuration variable. max indicates maximum number of connections.

    [chttpd]
    server_options = [{backlog, 128}, {acceptor_pool_size, 16}, {max, 4096}]

## 5.2.4. CouchDB

### DELETE operation

When you DELETE a document the database will create a new revision which contains the `_id` and `_rev` fields as well as the _deleted flag. This revision will remain even after a database compaction so that the deletion can be replicated. Deleted documents, like non-deleted documents, can affect view build times, PUT and DELETE request times, and the size of the database since they increase the size of the B+Tree. You can see the number of deleted documents in `database information`. If your use case creates lots of deleted documents (for example, if you are storing short-term data like log entries, message queues, etc), you might want to periodically switch to a new database and delete the old one (once the entries in it have all expired).

### Document’s ID

The db file size is derived from your document and view sizes but also on a multiple of your `_id` sizes. Not only is the `_id` present in the document, but it and parts of it are duplicated in the binary tree structure CouchDB uses to navigate the file to find the document in the first place. As a real world example for one user switching from 16 byte ids to 4 byte ids made a database go from 21GB to 4GB with 10 million documents (the raw JSON text when from 2.5GB to 2GB).

Inserting with sequential (and at least sorted) ids is faster than random ids. Consequently you should consider generating ids yourself, allocating them sequentially and using an encoding scheme that consumes fewer bytes. For example, something that takes 16 hex digits to represent can be done in 4 base 62 digits (10 numerals, 26 lower case, 26 upper case).

## 5.2.5. Views

### Views Generation

Views with the JavaScript query server are extremely slow to generate when there are a non-trivial number of documents to process. The generation process won’t even saturate a single CPU let alone your I/O. The cause is the latency involved in the CouchDB server and separate couchjs query server, dramatically indicating how important it is to take latency out of your implementation.

You can let view access be “stale” but it isn’t practical to determine when that will occur giving you a quick response and when views will be updated which will take a long time. (A 10 million document database took about 10 minutes to load into CouchDB but about 4 hours to do view generation).

In a cluster, “stale” requests are serviced by a fixed set of shards in order to present users with consistent results between requests. This comes with an availability trade-off - the fixed set of shards might not be the most responsive / available within the cluster. If you don’t need this kind of consistency (e.g. your indexes are relatively static), you can tell CouchDB to use any available replica by specifying `stable=false&update=false` instead of `stale=ok`, or `stable=false&update=lazy` instead of `stale=update_after`.

View information isn’t replicated - it is rebuilt on each database so you can’t do the view generation on a separate sever.

### Built-In Reduce Functions

If you’re using a very simple view function that only performs a sum or count reduction, you can call native Erlang implementations of them by simply writing `_sum` or `_count` in place of your function declaration. This will speed up things dramatically, as it cuts down on IO between CouchDB and the JavaScript query server. For example, as mentioned on the mailing list, the time for outputting an (already indexed and cached) view with about 78,000 items went down from 60 seconds to 4 seconds.

Before:

    {
        "_id": "_design/foo",
        "views": {
            "bar": {
                "map": "function (doc) { emit(doc.author, 1); }",
                "reduce": "function (keys, values, rereduce) { return sum(values); }"
            }
        }
    }

After:

    {
        "_id": "_design/foo",
        "views": {
            "bar": {
                "map": "function (doc) { emit(doc.author, 1); }",
                "reduce": "_sum"
            }
        }
    }

See also

Built-in Reduce Functions