Scylla FAQ


I’m not getting the level of performance I expected. What’s wrong?

Lower than expected performance can be a result of many factors, from HW (storage, CPU, network) to data modeling to the application layer. As a first step, make sure to have Scylla monitoring stack in place. Looking at the Scylla dashboard is the best way to look for bottlenecks. If you need our help, please follow How to Report a Performance Problem to share data securely.

Scylla is using all of my memory! Why is that? What if the server runs out of memory?

Scylla uses available memory to cache your data. Scylla knows how to dynamically manage memory for optimal performance; for example, if many clients connect to Scylla, it will evict some data from the cache to make room for these connections; when the connection count drops again, this memory is returned to the cache.

Can I limit Scylla to use less CPU and memory?

The --smp option (for instance, --smp 2) will restrict Scylla to a smaller number of CPUs. It will still use 100 % of those CPUs, but at least won’t take your system out completely. An analogous option exists for memory: -m.

Do I ever need to disable the Scylla cache to use less memory?

It is not possible to turn off the Scylla cache. But cache problems do not arise in normal operation. Scylla can use up to 50% of memory for cache, and will dynamically evict rows from the cache if they are too large. So the only possibility of getting an out of memory error is if a single row is bigger than 50% of the memory for a shard. This is (total_machine_memory / (number_of_lcores * 2)).

For a 64GB machine with 16 cores and hyperthreading enabled, you have 2GB per shard, of which the cache can use 1GB per shard. With such large rows, you will have other problems. We recommend staying with rows that are less than a few megabytes maximum size.

What are some of the techniques Scylla uses to achieve its performance?

Scylla tries to utilize all available resources (processor cores, memory, storage, and networking) by always operations in parallel and never blocking. If Scylla needs to read a disk block, it initiates the read and immediately moves on to another task. Later, when the read completes Scylla resumes the original task from where it left off. By never blocking, a high degree of concurrency is achieved, allowing all resources to be utilized to their limit. Read more on Scylla Architecture:

I understood Scylla underline Seastar framework use one thread per core, but I see more than two threads per core

Seastar creates an extra thread per core for blocking syscalls (like open()/ fsync() / close() ); this allows the Seastar reactor to continue executing while a blocking operation takes place. Those threads are usually idle, so they don’t contribute to significant context switching activity.

I’m seeing X compaction running in parallel on a single Scylla node. Is it normal?

Yes, for more than one reason:

  • each shard (core) will run its compactions independently, often at the same time,
  • each table will run its compactions independently, often at the same time
  • depending on the compaction strategy, more than one compaction can run in parallel. For example in Sized Tier Compaction Strategy (STCS), large sstable compaction can take time, allowing smaller sstable to be compacted at the same time

Setting io.conf configuration for HDD storage

As part of the Scylla setup process, iotune runs a short benchmark of your storage. When completed, it generates the /etc/scylla.d/io.conf configuration file. Note that iotune has known issues benchmarking HDD storage.


This section is not relevant in 2.3

Therefore, when using Scylla with HDD storage, it is recommended to use RAID0 on all of your available disks, and manually update the io.conf configuration file max-io-request parameter. This parameter sets the number of concurrent requests sent to the storage. The value for this parameter should be 3X (3 times) the number of your disks. For example, if you have 3 disks, you would set max-io-request=9.

Disk Space

Dropping a table does not reduce storage used by Scylla, how can I clean the disk from dropped tables?

scylla.yaml includes an auto_snapshot parameter; when true (it is by default), Scylla creates a snapshot for a table just before dropping it, as a safety measure. You can find the snapshot under the table sstable path, in the snapshots directory. For example, for dropped table users in keyspace mykeyspace:


As the snapshot take the same space as the dropped table, disk usage will remain the same. You can clean snapshots by using nodetool clearsnapshotteh. Read more on snapshot and snapshotclean


Will Scylla have a certain feature in an upcoming release?

Check the Status page for features scheduled for our GA release.

I want to try out new features. How do I enable experimental mode?

You need to add the line experimental: true to your scylla.yaml file.

  1. Launch the file in a text editor: $ vi /etc/scylla/scylla.yaml. (Alternately, on docker, it’s $ docker exec -it your_node vi /etc/scylla/scylla.yaml);

  2. Add the line experimental: true;

  3. Save the file and exit.

  4. Stop and restart the node.

    On RedHat Enterprise Linux, CentOS or Ubuntu:

    $ sudo systemctl restart scylla-server

    On Docker:

    $ docker stop <your_node> && docker start <your_node>

    Alternately, starting from Scylla 2.0, you can start Scylla for Docker with the experimental flag as follows:

    $ docker run --name <your_node> -d scylladb/scylla --experimental 1

You should now be able to use the experimental features available in your version of Scylla.

How do I check the current version of Scylla that I am running?

  • On a regular system or VM (running Ubuntu, CentOS, or RedHat Enterprise): $ scylla --version

Check the Operating System Support Guide for a list of supported operating systems and versions.

  • On a docker node: $ docker exec -it Node_Z scylla --version

Which version(s) of Apache Cassandra is Scylla compatible with? Will Scylla be compatible with future Cassandra versions?

Check the Cassandra Compatibility section for current and future Apache Cassandra release compatibility.

We plan to support the 2.2 series features and 3.0 as well, including materialized views.

Beyond 3.0, we will look at the features and see if they make sense for Scylla, but the general plan is to keep compatibility.

We plan to introduce extensions and enhancements. We’ll try to work with the Apache Cassandra community on new features. So that we don’t fragment the ecosystem, and will support switching back and forth between the two implementations while using the same drivers and application code.


Check and update Ubuntu 14.04 kernel

Running Scylla on Ubuntu 14.04 requires kernel 3.15 or later

  • To check your kernel version: $ uname -a
  • If your kernel is older than 3.15 then:
    • Check for available kernels: $ sudo apt-cache search linux-image
    • Install: $ sudo apt-get install linux-image-your_version_choice, for example linux-image-3.16.0
    • restart: $ sudo reboot now


Can I install Scylla on an Apache Cassandra server?

Scylla comes with its own version of the Apache Cassandra client tools, in the package scylla-tools. Trying to install it on a server with Cassandra already installed may result in something like:

Unpacking scylla-tools (1.0.1-20160411.b9fe89b-ubuntu1) ...
dpkg: error processing archive /var/cache/apt/archives/scylla-tools_1.0.1-20160411.b9fe89b-ubuntu1_all.deb (--unpack):
trying to overwrite '/usr/bin/nodetool', which is also in package cassandra 2.1.4

We recommend uninstalling Apache Cassandra before installing scylla-tools.

Which snitch or replication strategy should I use?

If you are creating a production cluster or if your cluster is going to have more than one data center you need to use a DC-aware snitch, e.g. GossipingPropertyFileSnitch or Ec2MutliRegionSnitch. You will also need to use a DC-aware replication strategy, e.g. NetworkTopologyStrategy.

However, if you are going to create your first cluster or want to try something simple, if your cluster is going to have a single data center then you may use a SimpleSnitch and then use a SimpleStrategy for your keyspaces.

Our general recommendation is to always use a NetworkTopologyStrategy and use Ec2XXX snitches on AWS based clusters and GossipingPropertyFileSnitch in all other cases.

A description of all snitch options we support may be found here: Snitches.

Note: trying to mix a SimpleSnitch with a DC-aware strategy or a DC-aware snitch with a SimpleStrategy may cause your cluster not to work as intended therefore we strongly discourage these types of configurations in general.

Not using a proper snitch-strategy combination may cause different types of errors.

For instance:

Unavailable: code=1000 [Unavailable exception] message="Cannot achieve consistency level for cl LOCAL_ONE. Requires 1, alive 0" info={'required_replicas': 1, 'alive_replicas': 0, 'consistency': 'LOCAL_ONE'}

While all Nodes are alive and are shown in a nodetool status report.

If you see this error you should always check that you are not using a SimpleSnitch in your cluster configuration in conjunction with some DC-aware replication strategy for a keyspace of a table you are failing to query.

When working with GossipingPropertyFileSnitch or Ec2MutliRegionSnitch you should edit the

For node using GossipingPropertyFileSnitch, the file should look like the following:

prefer_local= true

When the node is the Asia data center, on rack1 and to minimize BW usage for inter-datacenter, use the prefer_local

For Ec2MultiRegion the file should include the following information


This will create a suffix for the node location for example:


The problem may also arise if you are using some DC-aware snitch, e.g. Ec2MultiRegionSnitch, and a SimpleStrategy in a multi-DC cluster.

Please, make sure that both a snitch and a replication strategy of a keyspace are either both of a Simple kind or both are DC-aware.

After that, if you are using a DC-aware configuration, make sure that the replication strategy uses the proper data centers names. Verify the data centers names in your cluster using a nodetool status command.

Can I change the replication factor (a keyspace) on a live cluster?

Yes, but it will require running a full repair (or cleanup) to change the replica count of existing data:

  • Alter the replication factor for desired keyspace (using cqlsh for instance).
  • If you’re reducing the replication factor, run nodetool cleanup on the cluster to remove surplus replicated data. Cleanup runs on a per-node basis.
  • If you’re increasing the replication factor, run nodetool repair -full to ensure data is replicated according to the new configuration. Repair runs on a per-replica set basis. This is an intensive process that may result in adverse cluster performance. It’s highly recommended to do rolling repairs, as an attempt to repair the entire cluster at once will most likely swamp it. Note that you will need to run a full repair (-full) to make sure that already repaired sstables are not skipped.

Why can’t I set listen_address to listen to (all my addresses)?

Scylla is a gossip-based distributed system and listen_address is the address a node tells other nodes to reach it at. Telling other nodes “contact me on any of my addresses” is a bad idea; if different nodes in the cluster pick different addresses for you, Bad Things happen.

If you don’t want to manually specify an IP to listen_address for each node in your cluster (understandable!), leave it blank and Scylla will use InetAddress.getLocalHost() to pick an address. Then it’s up to you or your ops team to make things resolve correctly (/etc/hosts/, dns, etc).

More info

Where can I ask a question not covered here?

Two mailing lists.

  • scylladb-dev: Discuss the development of Scylla itself.
  • scylladb-users: Discuss using Scylla and developing client applications.

I delete data from Scylla, but disk usage stays the same. What gives?

Data you write to Scylla gets persisted to SSTables. Since SSTables are immutable, the data can’t actually be removed when you perform a delete, instead, a marker (also called a “tombstone”) is written to indicate the value’s new status. Never fear though, on the first compaction that occurs between the data and the tombstone, the data will be expunged completely and the corresponding disk space recovered.

What are seeds?

Seeds are used during startup to discover the cluster.

If you configure your nodes to refer some node as seed, nodes in your ring tend to send Gossip message to seeds more often than to non-seeds. In other words, seeds are worked as hubs of Gossip network. With seeds, each node can detect status changes of other nodes quickly.

Seeds are also referred by new nodes on bootstrap to learn about other nodes in the ring. When you add a new node to ring, you need to specify at least one live seed to contact. Once a node joins the ring, it learns about the other nodes so it doesn’t need seed on subsequent boot.

You can make a seed a node at any time. There is nothing special about seed nodes. If you list the node in seed list it is a seed

Seeds do not auto bootstrap (i.e. if a node has itself in its seed list it will not automatically transfer data to itself) If you want a node to do that, bootstrap it first and then add it to seeds later. If you have no data (new install) you do not have to worry about bootstrap at all.

Recommended usage of seeds:

  • Pick two (or more) nodes per data center as seed nodes.
  • Sync the seed list to all your nodes

Does single seed mean single point of failure?

The ring can operate or boot without a seed; however, you will not be able to add new nodes to the cluster. It is recommended to configure multiple seeds in production systems.

Is RAID0 required for Scylla? Why?

No, it is not required, but it is highly recommended when using Scylla with more than one drive. Scylla requires one drive for its data file and one drive for commit log (can be the same). If you want to take advantage of more than one drive, the easiest way to do so is set RAID0 (striped) across all of them. If you choose, scylla_setup will setup RAID0 for you on your selected drive, as well as XFS file system (recommended). Similarly, Scylla AMI on EC2 will mount automatically all available SSD drive in on RAID0 for you.

Should I use RAID for replications, such as RAID1, RAID4 or higher?

You can, but it is not recommended. Scylla clustering architecture already provides data replication across nodes and DCs. Adding another layer of replication in each node is redundant, slow down IO operation and reduce available storage. Want a higher level of replication? Increase the Replication Factor (RF) of relevant Keyspaces.

Is Nodetool Repair a Local (One Node) Operation or a Global (Full Cluster) Operation?

When running nodetool repair on a node, it performs a repair on every token range this node owns; this will also repair other nodes that share the same range.

If you wish to repair the entire cluster, it is recommended to run nodetool repair -pr on each node in the cluster, sequentially, or use the Scylla Manager.

in-memory tables

Is MV and SI supported for use with in-memory tables? If so, how will it affect the total memory size limitation?

You can make MV table an in-memory, as you would with every other table. It effects the total memory size allocation just like any other table. Make sure before you create any kind of in-memory table that its use case warrants the creation.

Can Scylla Enterprise in-memory tables be used without having a mirror file on disk?

No. In Scylla Enterprise 2018.1.7, im-memory tables are always persistent using an on-disk mirror file.

What happens if we apply an ALTER CQL command to change a table from/to in memory?

It goes in and out of in-memory, as expected. This is documented in the Scylla Docs.