Capacity planning

Background#

Capacity planning should be considered as part of the requirements of deploying QuestDB to forecast CPU, memory, network capacity, and a combination of these elements, depending on the expected demands of the system. This page describes configuring these system resources with example scenarios that align with both edge cases and common setup configurations.

All of the configuration settings referred to below with the exception of OS settings are configured in QuestDB by either a server.conf configuration file or as environment variables. For more details on applying configuration settings in QuestDB, refer to the configuration page.

CPU configuration#

In QuestDB, there are worker pools which can help separate CPU load between sub-systems. This section describes configuration strategies based on the forecast behavior of the database.

caution
  • Multiple workers must not have affinity (pinning) for the same core by ID. Although this is possible through manual configuration settings described below, this must be avoided as it can lead to indeterminate behavior.
  • When configuring the affinity of worker threads to CPUs and setting dedicated worker threads for tasks as described in the following sections, hyperthreading must be disabled in the system BIOS. If hyperthreading cannot be disabled, the core IDs should be ascertained and workers should not use adjacent cores to prevent overlapping work.

Shared workers#

The number of worker threads shared across the application can be configured as well as affinity to pin processes to specific CPUs by ID. Shared worker threads service SQL execution subsystems and, in the default configuration, every other subsystem. With the exception of SQL, every other subsystem can be configured to use their own worker threads. More information on these settings can be found on the shared workers configuration page.

Writer page size#

The default page size for writers is 16MB. In cases where there are a large number of small tables, using 16MB to write a maximum of 1Mb of data, for example, is a waste of OS resources. To changes the default value, set the append.page.size value in server.conf which is a rounded (ceiling) of the multiple of OS page sizes:

server.conf
cairo.sql.append.page.size=1

InfluxDB over TCP#

This section describes methods for optimizing ingestion of InfluxDB line protocol messages over TCP. For all configuration settings available for this subsystem, see the InfluxDB line over TCP configuration reference.

Message length#

When the message length is known, a starting point for optimization on ingestion is setting maximum measurement sizes and specifying buffer size for processing records:

server.conf
# max line length for measurements
line.tcp.max.measurement.size=2048
# buffer size to process messages at one time, cannot be less than measurement size
line.tcp.msg.buffer.size=2048

CPU affinity#

Given a single client sending data to QuestDB via InfluxDB line protocol over TCP, the following configuration can be applied which sets a dedicated worker and pins it with affinity to a CPU by core ID:

server.conf
line.tcp.worker.count=1
line.tcp.worker.affinity=1

Given two clients writing over TCP, multiple worker threads can be pinned to CPU cores by a comma-separated list of CPUs by core ID:

server.conf
line.tcp.worker.count=2
line.tcp.worker.affinity=1,2

Balancing work#

The following configuration settings may be applied in relation to balancing unequal distribution of work across writer threads. The number of updates per load balance refers to the number of updates (per table) between attempts to redistribute the load between writer workers.

The maximum load ratio defaults to 1.9 and this figure is the ratio of least busy worker to most busy worker. This value of 1.9 means redistribution of work will occur when a thread is performing almost twice as much work as a thread with least amount of work.

server.conf
# for balancing work when writers spread across multiple tables
line.tcp.n.updates.per.load.balance=2048
# Maximum load ratio (max loaded worker/min loaded worker)
line.tcp.max.load.ratio=1.9

Committing records#

These two configuration settings are relevant for maintenance jobs which commit uncommitted records to tables. This maintenance of committing records will occur if either:

  • the max number of uncommitted rows is hit (default of 1000) or
  • when the maintenance job interval is reached
server.conf
# commit when this number of uncommitted records is reached
line.tcp.max.uncommitted.rows=1000
# commit uncommitted rows when this timer is reached
line.tcp.maintenance.job.interval=1000

InfluxDB over UDP#

Given a single client sending data to QuestDB via InfluxDB line protocol over UDP, the following configuration can be applied which dedicates a thread for a UDP writer and specifies a CPU core by ID:

server.conf
line.udp.own.thread=true
line.udp.own.thread.affinity=1

Postgres#

Given clients sending data to QuestDB via Postgres interface, the following configuration can be applied which sets a dedicated worker and pins it with affinity to a CPU by core ID:

server.conf
pg.worker.count=4
pg.worker.affinity=1,2,3,4

Storage#

The following section describes aspects to be considered regarding the storage of data.

Partitioning#

When creating tables, a partitioning strategy is recommended in order to be able to enforce a data retention policy to save disk space, and for optimizations on the number of concurrent file reads performed by the system. For more information on this topic, see the following resources:

  • partitions page which provides a general overview of this concept
  • data retention guide provides further details on partitioning tables with examples on how to drop partitions by time range

Records per partition

The number of records per partition should factor into the partitioning strategy (DAY, MONTH, YEAR). Having too many records per partition or having too few records per partition and having query operations across too many partitions has the result of slower query times. A general guideline is that roughly between 1 million and 100 million records is optimal per partition.

Choosing a schema#

This section provides some hints for choosing the right schema for a dataset based on the storage space that types occupy in QuestDB.

Symbols#

Symbols are a data type that is recommended to be used for strings that are repeated often in a dataset. The benefit of using this data type is lower storage requirements than regular strings and faster performance on queries as symbols are internally stored as int values.

info

Only symbols can be indexed in QuestDB. Although multiple indexes can be specified for a table, there would be a performance impact on the rate of ingestion.

The following example shows the creation of a table with a symbol type that has multiple options passed for performance optimization.

CREATE TABLE my_table(
symb SYMBOL capacity 256 nocache index capacity 1048576,
ts TIMESTAMP, s STRING
) timestamp(ts) PARTITION BY DAY;

This example adds a symbol type with:

  • capacity specified to estimate how many unique symbols values to expect
  • caching disabled which allows dealing with larger value counts
  • index for the symbol column with a storage block value

A full description of the options used above for symbol types can be found in the CREATE TABLE page.

Numbers#

The storage space that numbers occupy can be optimized by choosing byte, short, and int data types appropriately. When values are not expected to exceed the limit for that particular type, savings on disk space can be made.

typestorage per valuenumeric range
byte8 bits-128 to 127
short16 bits-32768 to 32767
int32 bits-2147483648 to 2147483647

Network Configuration#

For InfluxDB line, Postgres wire and HTTP protocols, there are a set of configuration settings relating to the number of clients that may connect, the internal IO capacity and connection timeout settings. These settings are configured in the server.conf file in the format:

<protocol>.net.<config>

Where <protocol> is one of:

  • http - HTTP connections
  • pg - Postgres wire protocol
  • line.tcp - InfluxDB line protocol over TCP

And <config> is one of the following settings:

keydescription
active.connection.limitThe number of simultaneous connections to the server. This value is intended to control server memory consumption.
event.capacityInternal IO event queue capacity (EPoll, KQqueu, Select). Size of these queues must be larger than active.connection.limit.
io.queue.capacityInternal IO queue of the server. The size of this queue must be larger than the active.connection.limit. A queue size smaller than active connection max will substantially slow down the server by increasing wait times. A queue larger than connection max reduces wait time to 0.
idle.connection.timeoutConnection idle timeout in milliseconds. Connections are closed by the server when this timeout lapses.
interest.queue.capacityInternal queue size. This is also related to active.connection.limit in a way that sizes larger than connection max remove any waiting.
listen.backlogBacklog argument value for listen() call.
snd.buf.sizeMaximum send buffer size on each TCP socket. If value is -1 socket send buffer remains unchanged from OS default.
rcv.buf.sizeMaximum receive buffer size on each TCP socket. If value is -1, the socket receive buffer remains unchanged from OS default.

For example, the default network configuration for InfluxDB line protocol is the following:

server.conf InfluxDB line protocol network defaults
line.tcp.net.active.connection.limit=10
line.tcp.net.bind.to=0.0.0.0:9009
line.tcp.net.event.capacity=1024
line.tcp.net.io.queue.capacity=1024
line.tcp.net.idle.timeout=0
line.tcp.net.interest.queue.capacity=1024
line.tcp.net.listen.backlog=50000
line.tcp.net.recv.buf.size=-1

For reference on the defaults of the http and pg protocols, refer to the server configuration page

OS configuration#

This section describes approaches for changing system settings on the host QuestDB is running on when system limits are reached due to maximum open files or virtual memory areas. QuestDB passes operating system errors to its logs unchanged and as such, changing the following system settings should only be done in response to such OS errors.

Maximum open files#

The storage model of QuestDB has the benefit that most data structures relate closely to the file system, with columnar data being stored in it's own .d file per partition. In edge cases with extremely large tables, the number of open files may hit a user or system-wide maximum limit and can cause unpredictable behavior.

The following commands allow for checking current user and system limits for maximum number of open files:

checking ulimit
# Soft limit
ulimit -Sn
# Hard limit
ulimit -Hn

Setting system-wide open file limit:

To increase this setting and have the configuration persistent, the limit on the number of concurrently open files can be changed in /etc/sysctl.conf:

/etc/sysctl.conf
fs.file-max=100000

To confirm that this value has been correctly configured, reload sysctl and check the current value:

# reload configuration
sysctl -p
# query current settings
sysctl fs.file-max

Max virtual memory areas limit#

If the host machine has insufficient limits of map areas, this may result in out of memory exceptions. To increase this value and have the configuration persistent, mapped memory area limits can be changed in /etc/sysctl.conf:

/etc/sysctl.conf
vm.max_map_count=262144

Each mapped area needs kernel memory and it's recommended to have around 128 bytes available per 1 map count.

# reload configuration
sysctl -p
# query current settings
cat /proc/sys/vm/max_map_count