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VAST v2.3

· 4 min read
Dominik Lohmann

VAST v2.3 is now available, which introduces an automatic data defragmentation capability.

Automatic Rebuilds

VAST server processes now continuously rebuild partitions in the background. The following diagram visualizes what happens under the hood:

Rebuild Rebuild

Rebuilding kicks in when a partition has the following properties:

  1. Outdated: if a partitions does not have the latest partition version, it may not enjoy the latest features and optimizations. It makes it also faster to adopt VAST versions that include breaking changes in the storage layout. Therefore, VAST rebuilds outdated partitions to bring them into the most recent state.

  2. Undersized: numerous small partitions can cause fragmentation in the catalog, causing higher memory consumption, larger database footprint, and slower queries. Rebuilding merges undersized partitions, thereby defragmenting the system. This reduces the resource footprint and makes queries faster.

To enable automatic rebuilding, set the new vast.automatic-rebuild option.

# Control automatic rebuilding of partitions in the background for
# optimization purposes. The given number controls how many rebuilds to run
# concurrently, and thus directly controls the performance vs. memory and CPU
# usage trade-off. Set to 0 to disable. Defaults to 1.
automatic-rebuild: 1

Now that we have an LSM-style merge operation of partitions, we reduced the partition cutoff timeout to 5 minutes from 1 hour by default (controlled through the option This reduces the risk of data loss in case of a crash. This comes in handy in particular for low-volume data sources that never exhaust their capacity.

Optional Partition Indexes

Historically, VAST evolved from a special-purpose bitmap indexing system into a general-purpose telemetry engine for security data. Today, VAST has a two-tiered indexing architecture with sparse sketch structures at the top, followed by a second layer of dense indexes. As of this release, it is possible to disable this second layer.

The space savings can be substantial based on the size of your index. For example, if the first layer of indexing always yields highly selective results, then it the dense indexes do not provide a lot of value. One scenario would be retro-matching: if you only do IoC-style point queries, they will be most likely covered well by the sketches. If you do not have selective queries, the dense index is not helping much anyway, since you need access the base data anyway. A really good use case for the indexes when your have a scatterd data access patterns, i.e., highly selective results within a partition, but a result that spans many disparate partitions.

In a simplified model, VAST performs three steps when executing a query:

  1. Send the query to the catalog, which maintains VAST's partitions, and ask it for a list of candidate partitions. The catalog maintains the first tier of sparse indexes, currently one per partition.

  2. Send the query to all candidate partitions in parallel, each of which contains dense indexes for all fields in the partition's schema. The index lookup yields a set of candidate records IDs within the partition.

  3. Send the query to all candidate partition's stores, provided the index lookup yielded record IDs. Then evaluating the query against the candidate events and return the result.

Here's how you can configure a partition index to be disabled:

# Don't create partition indexes the suricata.http.http.url field.
- targets:
- suricata.http.http.url
partition-index: false
# Don't create partition indexes for fields of type addr.
- targets:
- :addr
partition-index: false

Improved Responsiveness Under High Load

Two small changes improve VAST's behavior under exceptionally high load.

First, the new vast.connection-timeout option allows for modifying the default client-to-server connection timeout of 10 seconds. Previously, if a VAST server was too busy to respond to a new client within 10 seconds, the client simply exited with an unintelligable request_timeout error message. Here's how you can set a custom timeout:

# The timeout for connecting to a VAST server. Set to 0 seconds to wait
# indefinitely.
connection-timeout: 10s

The option is additionally available under the environment variable VAST_CONNECTION_TIMEOUT and the --connection-timeout command-line option.

Second, we improved the operability of VAST servers under high load from automated low-priority queries. We noticed that when spawning thousands of automated retro-match queries that compaction would stall and make little visible progress, risking the disk running full or no longer being compliant with GDPR-related policies enforced by compaction.

To ensure that compaction's internal and regular user-issued queries work as expected even in this scenario, VAST now considers queries issued with --low-priority, with even less priority compared to regular queries (down from 33.3% to 4%) and internal high-priority queries used for rebuilding and compaction (down from 12.5% to 1%).