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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 operating 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 thought that Scylla’s underlying Seastar framework uses one thread per core, but I see more than two threads per core. Why?

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.

Do I need to configure swap on a Scylla node?

Yes, configuring swap on a Scylla node is recommended. swap size should be set to either total_mem/3 or 16GB - lower of the two.

total_mem is the total size of the nodes memory.

For example:

  • If the node total_mem is 18GB swap size should be set to 6GB.

  • If the node total_mem is 240GB swap size should be set to 16GB.

Swap can be set up in several ways. One way to set up swap is detailed in the KB Article How to Set up a Swap Space.

After upgrade from Scylla 2.1 and older, scylla_reactor_utilization metrics is at high percentage (high CPU utilization is observed). Why does this happen?

Scylla 2.2 enables the compaction automatic controller which was not present prior to version 2.2. What this means is that in Scylla 2.1 (and earlier) the system waits for 4 SSTables to be present in the same tier before starting a compaction. In Scylla 2.2 (and later), compactions can be controlled to not impact the workload. This means that workloads which have been considered as backlog in Scylla 2.1 and earlier, in Scylla 2.2 and later are not.

My query does not return any or some of the data? What happened?

If you are using a time range in the query, refer to the solution in the troubleshooting document, Time Range Queries Do Not Return Some or All of the Data.

DESC SCHEMA shows that I am using many materialized views (MVs) when I know I only added Secondary Indexes (SI). Why are there MVs in my schema?

As SI is built ontop of MV, you can expect to see MV in your schema. There is nothing wrong with your system. More information on Global Secondary Indexes.

Using the Java driver SimpleStatements are slow. Why does this happen?

The Java driver’s SimpleStatement is token unaware by default. This means that requests sent out will reach the Controller node before it is known which shard it’s supposed to access. We suggest using PreparedStatements instead.

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 in the snapshots directory, under the table SSTable. 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 clearsnapshot. Read more on snapshot and clearsnapshot


Will Scylla have a certain feature in an upcoming release?

Check the Roadmap 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

Is Scylla Apache Cassandra compatible? Is API / interface X compatible?

Check the Cassandra Compatibility section for compatibility matrix.

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.

I am upgrading my nodes to a version that uses a newer SSTable format, when will the nodes start using the new SSTable format?

The new “mc” SSTable format is supported in Scylla 3.0 and later. Scylla only starts using the newer format when every node in the cluster is capable to generate it. Therefore, only when all nodes in the cluster are upgraded the new format is used.


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 Ec2MultiRegionSnitch. 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'}

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 Ec2MultiRegionSnitch 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 <updated Keyspace> on the keyspace you modified to remove surplus replicated data. Cleanup runs on a per-node basis.

  • If you’re increasing the replication factor, refer to How to Safely Increase the RF

  • Note that you need to provide the keyspace namr. If you do not, the cleanup or repair operation runs on all keyspaces for the specific node.

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).

What is the best scenario to add a node to a multi availability zone (AZ)?

If using three node cluster, with RF=3, each node located on a different availability zone (AZ).

For example:

Datacenter: DC1
--  Address        Load       Tokens  Owns (effective)                         Host ID         Rack
UN  118.82 KB  256     33.6%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   A1
UN  111.82 KB  256     33.1%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   B1
UN  114.82 KB  256     33.3%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   C1

All nodes holds 100% of the data. If needed to add a single node to the cluster (scale out), the cluster will become imbalance. Because the single additional node will split the tokens only with the existing node in the same AZ.


This is only an example, if having more nodes or different RF the number of nodes may be different.

The token distribution will be:

AZ A1 node A: 100% of the data
AZ B1 node B: 100% of the data
AZ C1 node C: 50% of the data
AZ C1 node D: 50% of the data

The solution is to add a node in each AZ.

Datacenter: DC1
--  Address        Load       Tokens  Owns (effective)                         Host ID         Rack
UN  118.82 KB  256     16.6%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   A1
UN  111.82 KB  256     16.1%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   B1
UN  114.82 KB  256     16.3%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   C1
UN  118.82 KB  256     16.6%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   A1
UN  111.82 KB  256     16.1%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   B1
UN  114.82 KB  256     16.3%             8d5ed9f4-7764-4dbd-bad8-43fddce94b7c   C1

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 deleted data from Scylla, but disk usage stays the same. Why?

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 the 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 to contact the seeds on subsequent boots.

You can make a seed a non-seed node at any time. There is nothing special about seed nodes. If you list the node in seed list it is a seed, and if you remove it it is not.

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 automatically mount all available SSD drives in RAID0.

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, slows down I/O operation and reduces available storage. Want a higher level of replication? Increase the Replication Factor (RF) of relevant Keyspaces.

Can I use JBOD and not use RAID0?

JBOD is not supported by Scylla.

JBOD may be a reasonable solution for Cassandra because it rebuilds nodes very slowly. As this is not an issue for Scylla, it’s more efficient to use RAID.

Explanation: There are two types of deployment when multiple disks exist. In the JBOD case, each disk is an isolated filesystem. I/O isn’t stripped and thus performance can be slower than that of RAID. In addition, as the free space isn’t shared, a single disk can be full while the others are available.

The benefit of JBOD vs RAID is that it isolates failures to individual disk and not the entire node. However, Scylla rebuilds nodes quickly and thus it is not an issue when rebuilding an entire node.

As a result, it is much more advantageous to use RAID with Scylla

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, in-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.

Can I change the coredump mount point?

Yes, by edit sysctl.d.


  1. Create /etc/sysctl.d/99-scylla-coredump.conf (this file exists by default in Scylla AMI).

  2. Open the 99-scylla-coredump.conf file.

  3. Add the following line kernel.core_pattern=|/<path>/<coredump_directory> %p %u %g %s %t %e"

For example:

kernel.core_pattern=|/home/centos/core/ %p %u %g %s %t %e"
  1. Run sysctl -p /etc/sysctl.d/99-scylla-coredump.conf

Do I need to run a tool like upgradesstables when moving to a new format?

Unlike Apache Cassandra, Scylla does not ship with upgradesstables, a tool that converts SSTables to newer formats. When upgrading to a new table format, Scylla can still continue to read from the old format. Having this option, ensures a smoother transition and upgrade. New writes use the new format and reads will use both formats until the old tables are removed. If you want to purge all of the old SSTables in a single step, generate a compaction with nodetool compact follow by nodetool cleanup to remove no longer needed token ranges that belong to that node.


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