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+<!--
+title: "Database"
+description: "The Netdata Agent leverages multiple, user-configurable time-series databases that use RAM and/or disk to store metrics on any type of node."
+custom_edit_url: https://github.com/netdata/netdata/edit/master/database/README.md
+-->
+
+# Database
+
+Netdata is fully capable of long-term metrics storage, at per-second granularity, via its default database engine
+(`dbengine`). But to remain as flexible as possible, Netdata supports a number of types of metrics storage:
+
+1. `dbengine`, (the default) data are in database files. The [Database Engine](/database/engine/README.md) works like a
+ traditional database. There is some amount of RAM dedicated to data caching and indexing and the rest of the data
+ reside compressed on disk. The number of history entries is not fixed in this case, but depends on the configured
+ disk space and the effective compression ratio of the data stored. This is the **only mode** that supports changing
+ the data collection update frequency (`update_every`) **without losing** the previously stored metrics. For more
+ details see [here](/database/engine/README.md).
+
+2. `ram`, data are purely in memory. Data are never saved on disk. This mode uses `mmap()` and supports [KSM](#ksm).
+
+3. `save`, data are only in RAM while Netdata runs and are saved to / loaded from disk on Netdata
+ restart. It also uses `mmap()` and supports [KSM](#ksm).
+
+4. `map`, data are in memory mapped files. This works like the swap. Keep in mind though, this will have a constant
+ write on your disk. When Netdata writes data on its memory, the Linux kernel marks the related memory pages as dirty
+ and automatically starts updating them on disk. Unfortunately we cannot control how frequently this works. The Linux
+ kernel uses exactly the same algorithm it uses for its swap memory. Check below for additional information on
+ running a dedicated central Netdata server. This mode uses `mmap()` but does not support [KSM](#ksm).
+
+5. `none`, without a database (collected metrics can only be streamed to another Netdata).
+
+6. `alloc`, like `ram` but it uses `calloc()` and does not support [KSM](#ksm). This mode is the fallback for all
+ others except `none`.
+
+You can select the memory mode by editing `netdata.conf` and setting:
+
+```conf
+[global]
+ # dbengine (default), ram, save (the default if dbengine not available), map (swap like), none, alloc
+ memory mode = dbengine
+
+ # the directory where data are saved
+ cache directory = /var/cache/netdata
+```
+
+## Running Netdata in embedded devices
+
+Embedded devices usually have very limited RAM resources available.
+
+There are 2 settings for you to tweak:
+
+1. `update every`, which controls the data collection frequency
+2. `history`, which controls the size of the database in RAM (except for `memory mode = dbengine`)
+
+By default `update every = 1` and `history = 3600`. This gives you an hour of data with per second updates.
+
+If you set `update every = 2` and `history = 1800`, you will still have an hour of data, but collected once every 2
+seconds. This will **cut in half** both CPU and RAM resources consumed by Netdata. Of course experiment a bit. On very
+weak devices you might have to use `update every = 5` and `history = 720` (still 1 hour of data, but 1/5 of the CPU and
+RAM resources).
+
+You can also disable [data collection plugins](/collectors/README.md) you don't need. Disabling such plugins will also free both
+CPU and RAM resources.
+
+## Running a dedicated central Netdata server
+
+Netdata allows streaming data between Netdata nodes. This allows us to have a central Netdata server that will maintain
+the entire database for all nodes, and will also run health checks/alarms for all nodes.
+
+For this central Netdata, memory size can be a problem. Fortunately, Netdata supports several memory modes. **One
+interesting option** for this setup is `memory mode = map`.
+
+### map
+
+In this mode, the database of Netdata is stored in memory mapped files. Netdata continues to read and write the database
+in memory, but the kernel automatically loads and saves memory pages from/to disk.
+
+**We suggest _not_ to use this mode on nodes that run other applications.** There will always be dirty memory to be
+synced and this syncing process may influence the way other applications work. This mode however is useful when we need
+a central Netdata server that would normally need huge amounts of memory. Using memory mode `map` we can overcome all
+memory restrictions.
+
+There are a few kernel options that provide finer control on the way this syncing works. But before explaining them, a
+brief introduction of how Netdata database works is needed.
+
+For each chart, Netdata maps the following files:
+
+1. `chart/main.db`, this is the file that maintains chart information. Every time data are collected for a chart, this
+ is updated.
+2. `chart/dimension_name.db`, this is the file for each dimension. At its beginning there is a header, followed by the
+ round robin database where metrics are stored.
+
+So, every time Netdata collects data, the following pages will become dirty:
+
+1. the chart file
+2. the header part of all dimension files
+3. if the collected metrics are stored far enough in the dimension file, another page will become dirty, for each
+ dimension
+
+Each page in Linux is 4KB. So, with 200 charts and 1000 dimensions, there will be 1200 to 2200 4KB pages dirty pages
+every second. Of course 1200 of them will always be dirty (the chart header and the dimensions headers) and 1000 will be
+dirty for about 1000 seconds (4 bytes per metric, 4KB per page, so 1000 seconds, or 16 minutes per page).
+
+Hopefully, the Linux kernel does not sync all these data every second. The frequency they are synced is controlled by
+`/proc/sys/vm/dirty_expire_centisecs` or the `sysctl` `vm.dirty_expire_centisecs`. The default on most systems is 3000
+(30 seconds).
+
+On a busy server centralizing metrics from 20+ servers you will experience this:
+
+![image](https://cloud.githubusercontent.com/assets/2662304/23834750/429ab0dc-0764-11e7-821a-d7908bc881ac.png)
+
+As you can see, there is quite some stress (this is `iowait`) every 30 seconds.
+
+A simple solution is to increase this time to 10 minutes (60000). This is the same system with this setting in 10
+minutes:
+
+![image](https://cloud.githubusercontent.com/assets/2662304/23834784/d2304f72-0764-11e7-8389-fb830ffd973a.png)
+
+Of course, setting this to 10 minutes means that data on disk might be up to 10 minutes old if you get an abnormal
+shutdown.
+
+There are 2 more options to tweak:
+
+1. `dirty_background_ratio`, by default `10`.
+2. `dirty_ratio`, by default `20`.
+
+These control the amount of memory that should be dirty for disk syncing to be triggered. On dedicated Netdata servers,
+you can use: `80` and `90` respectively, so that all RAM is given to Netdata.
+
+With these settings, you can expect a little `iowait` spike once every 10 minutes and in case of system crash, data on
+disk will be up to 10 minutes old.
+
+![image](https://cloud.githubusercontent.com/assets/2662304/23835030/ba4bf506-0768-11e7-9bc6-3b23e080c69f.png)
+
+To have these settings automatically applied on boot, create the file `/etc/sysctl.d/netdata-memory.conf` with these
+contents:
+
+```conf
+vm.dirty_expire_centisecs = 60000
+vm.dirty_background_ratio = 80
+vm.dirty_ratio = 90
+vm.dirty_writeback_centisecs = 0
+```
+
+There is another memory mode to help overcome the memory size problem. What is **most interesting for this setup** is
+`memory mode = dbengine`.
+
+### dbengine
+
+In this mode, the database of Netdata is stored in database files. The [Database Engine](/database/engine/README.md)
+works like a traditional database. There is some amount of RAM dedicated to data caching and indexing and the rest of
+the data reside compressed on disk. The number of history entries is not fixed in this case, but depends on the
+configured disk space and the effective compression ratio of the data stored.
+
+We suggest to use **this** mode on nodes that also run other applications. The Database Engine uses direct I/O to avoid
+polluting the OS filesystem caches and does not generate excessive I/O traffic so as to create the minimum possible
+interference with other applications. Using memory mode `dbengine` we can overcome most memory restrictions. For more
+details see [here](/database/engine/README.md).
+
+## KSM
+
+Netdata offers all its round robin database to kernel for deduplication (except for `memory mode = dbengine`).
+
+In the past KSM has been criticized for consuming a lot of CPU resources. Although this is true when KSM is used for
+deduplicating certain applications, it is not true with netdata, since the Netdata memory is written very infrequently
+(if you have 24 hours of metrics in netdata, each byte at the in-memory database will be updated just once per day).
+
+KSM is a solution that will provide 60+% memory savings to Netdata.
+
+### Enable KSM in kernel
+
+You need to run a kernel compiled with:
+
+```sh
+CONFIG_KSM=y
+```
+
+When KSM is enabled at the kernel is just available for the user to enable it.
+
+So, if you build a kernel with `CONFIG_KSM=y` you will just get a few files in `/sys/kernel/mm/ksm`. Nothing else
+happens. There is no performance penalty (apart I guess from the memory this code occupies into the kernel).
+
+The files that `CONFIG_KSM=y` offers include:
+
+- `/sys/kernel/mm/ksm/run` by default `0`. You have to set this to `1` for the
+ kernel to spawn `ksmd`.
+- `/sys/kernel/mm/ksm/sleep_millisecs`, by default `20`. The frequency ksmd
+ should evaluate memory for deduplication.
+- `/sys/kernel/mm/ksm/pages_to_scan`, by default `100`. The amount of pages
+ ksmd will evaluate on each run.
+
+So, by default `ksmd` is just disabled. It will not harm performance and the user/admin can control the CPU resources
+he/she is willing `ksmd` to use.
+
+### Run `ksmd` kernel daemon
+
+To activate / run `ksmd` you need to run:
+
+```sh
+echo 1 >/sys/kernel/mm/ksm/run
+echo 1000 >/sys/kernel/mm/ksm/sleep_millisecs
+```
+
+With these settings ksmd does not even appear in the running process list (it will run once per second and evaluate 100
+pages for de-duplication).
+
+Put the above lines in your boot sequence (`/etc/rc.local` or equivalent) to have `ksmd` run at boot.
+
+## Monitoring Kernel Memory de-duplication performance
+
+Netdata will create charts for kernel memory de-duplication performance, like this:
+
+![image](https://cloud.githubusercontent.com/assets/2662304/11998786/eb23ae54-aab6-11e5-94d4-e848e8a5c56a.png)
+
+[![analytics](https://www.google-analytics.com/collect?v=1&aip=1&t=pageview&_s=1&ds=github&dr=https%3A%2F%2Fgithub.com%2Fnetdata%2Fnetdata&dl=https%3A%2F%2Fmy-netdata.io%2Fgithub%2Fdatabase%2FREADME&_u=MAC~&cid=5792dfd7-8dc4-476b-af31-da2fdb9f93d2&tid=UA-64295674-3)](<>)