Using Snowflake stage volumes with services¶

Snowflake supports various storage volume types for your application containers, including internal stage, local storage, memory storage, and block storage volumes. This section explains how to configure volumes and volume mounts for internal stages. A An internal stage volume is a volume configured to use a Snowflake stage as persistent storage.

With stage volumes your service can access an internal stage’s objects as if they are files on your file system, simplifying your service code compared to using a Snowflake driver and GET and PUT SQL commands to access these objects. Stage volumes can also perform better for scenarios with streaming reads or writes of large data files.

If you file system operations can easily be translated to streaming GET and PUT operations, then Stage volumes will work well for your scenario. If your application needs to rename or move files, modify existing files, or perform file system based locking, then stage volume is not a good fit for your workload.

Note

There are currently two implementations of stage volumes; a generally available version and a deprecated version. Snowflake recommends that you use the generally available version for new services and that you migrate your existing applications from the deprecated version.

The stage volume implementation streams file contents directly to and from cloud storage, ensuring that you always get the latest contents. Consider the following points when you use a stage volume:

A stage volume is optimized for large, sequential reads and writes, providing strong performance for these access patterns. For best results, read and write data in large, contiguous chunks.
Reads always return the latest data, which lets data sharing occur between services.
Random writes or file appends aren’t supported. Attempting these operations results in an error. Snowflake recommends that you use block storage volumes for these workloads.

Configure a Snowflake stage as a storage volume in a service specification¶

To create a service where service containers use a stage volume, you perform two steps to specify the required settings in the service specification:

Define a stage volume that identifies the Snowflake stage to use as storage volume.
Specify where to mount the stage volume in your application container.

Step 1: Define a stage volume¶

To define a stage volume, add the spec.volumes field in the service specification as shown in the following example. The example shows only the minimum required fields:

spec:
  containers:
    ..
  volumes:
  - name: <name>
    source: stage
    stageConfig:
       name: <stage_name>

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The following list defines the fields from the example:

name: Provides the name of the volume.
source: Identifies the type of the volume (stage).
stageConfig.name: Identifies the Snowflake internal stage or folder on a stage to mount; for example @my_stage, @my_stage/folder, or @my_db.my_schema.my_stage/folder/nestedfolder. Double quotes must surround this value.

You can include the following optional fields in stageConfig:

stageConfig.resources field: The Snowflake component that provides the mounted stage volume requires CPU and memory resources. Use this field to specify these CPU and memory requirements, similar to the resource specifications for your application containers. For more information, see containers.resources field fields. If this field isn’t specified, the following default resource settings apply:
- resources.requests.cpu: 0
- resources.requests.memory: 0.5Gi
- resources.limits.cpu: 0.5
- resources.limits.memory: 1Gi
For most applications with typical data traffic to stage volumes, you don’t need to set this field, because the default resource settings should be sufficient. However, if your application performs a high volume of reads and writes, the default settings might lead to performance constraints or read/write failures. For more information, see Common guidelines for both implementations of stage volumes.

To avoid these problems, check the CPU and memory metrics for the container (stage-mount-v2-sidecar-<stage-volume-name>). Snowflake adds this container to your service that provides the implementation of the stage volume you configured. This lets you to see exactly what resources your stage volume is using and determine if it is constrained by CPU or memory. Use these metrics to update the CPU and memory limits as needed.
stageConfig.metadataCache field: If your application frequently retrieves file metadata or lists files on a Snowflake stage in a loop, and you don’t expect the data to change often, you can enable metadata caching for the Snowflake stage storage volume to significantly improve performance. The cache stores this metadata for a specified time period, after which it is cleared. If the application then tries to access the metadata, Snowflake refreshes the cache. Use the hours, minutes, and seconds units to specify the metadataCache. For example 90s, 5m, 1h, 1h30m, 1h30m45s. If not specified, there is no caching.

Note

Configure metadata caching only when the data in your Snowflake stage doesn’t change for service lifetime; for example, a service that has read-only workloads that need to work on a static set of data in the stage. Don’t configure metadata caching for workloads where data in your Snowflake stage is updated while the service is running.

The following spec.volumes field defines a Snowflake stage volume. The field includes the optional stageConfig fields:

spec:
  containers:
    ..
  volumes:
  - name: <name>
    source: stage
    stageConfig:
        name: <stage_name>
        metadataCache: 1h
        resources:
            requests:
                cpu: 0.35
                memory: 0.4Gi
            limits:
                cpu: 2.0
                memory: 1Gi

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Step 2: Specify where to mount the stage volume in the container¶

After you define a Snowflake stage storage volume by adding the spec.volumes field, use the spec.containers.volumeMounts field to describe where to mount the stage volume in your application containers, as shown in the following example:

spec:
  containers:
  name: ...
  image: ...
  volumeMounts:
  - name: <name>
    mountPath: <absolute_directory_path>

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The information you provide in this field is consistent across all supported storage volume types and applies to both implementations of stage volumes.

Example¶

In the example service specification, the app container mounts an internal stage @ai_models_stage:

spec:
  containers:
  - name: app
    image: <image1-name>
    volumeMounts:
    - name: ai-models
      mountPath: /opt/ai-model
  volumes:
  - name: ai-models
    source: stage
    stageConfig:
      name: "@ai_models_stage"

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Access control requirements¶

The service’s owner role is the role that is used to create the service. It is also the role the services use when interacting with Snowflake. This owner role determines the permissions granted to application containers for accessing a mounted stage. The owner role must have the READ privilege on the stage.

If the owner role doesn’t have the WRITE privilege on a stage, the mount for that stage is read-only. That is, the containers can only read the files from the stage. The owner role needs the WRITE privilege on a stage for the stage mount to support both read and write.

About the deprecated implementation¶

The deprecated stage-volume implementation uses a shared cache for reads and writes. Although this works well for some scenarios, you can’t control whether data is read from the cache or directly from the stage, which might not be suitable for all applications. Additionally, when you use the cache for reads and writes, this can introduce performance overhead.

Migrating code from the deprecated implementation¶

The newer implementation replaces the deprecated implementation, with the following behavioral changes:

The newer stage-volume implementation streams file contents directly to and from cloud storage, ensuring that you always get the latest contents. This provides predictable behavior and significantly faster throughput. The deprecated stage-volume implementation caches chunks of file data, making it difficult to know if you are reading the latest data.
Random read performance might be lower with the new implementation because of the removal of caching. However, without a local disk cache, consistency across volumes is improved. File changes are written directly to the backing stage when the file is closed, with no local disk buffering.
Reads always return the latest data, making this configuration better for sharing data between services.
Random writes or file appends aren’t supported. Attempting these operations results in an error. Snowflake recommends that you use block storage volumes for these workloads.

Specify a Snowflake stage volume in a service specification (deprecated)¶

To create a service where service containers use Snowflake stage volume, specify the required settings in the service specification as shown in the following steps:

To specify the stage volume, use the spec.volumes field as shown in the following example:
```
volumes:
- name: <name>
  source: <stage_name>
```
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The following fields are required:
- name: The name of the volume.
- source: The Snowflake internal stage or folder on the stage to mount; for example @my_stage, @my_stage/folder. Quotes must surround this value.
To describe where to mount the stage volume in your application containers, use the spec.containers.volumeMounts field, as shown in the following example:
```
volumeMounts:
- name: <name>
  mountPath: <absolute_directory_path>
```
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The information you provide in this field is consistent across all supported storage volume types and applies to both implementations of stage volumes.

Example (deprecated)¶

In the example service specification, the app container mounts an internal stage @model_stage by using the deprecated stage volume implementations:

spec:
  containers:
  - name: app
    image: <image1-name>
    volumeMounts:
    - name: models-legacy
      mountPath: /opt/model-legacy
  volumes:
  - name: models-legacy
    source: "@model_stage"

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The volumeMounts field specifies where inside the container to mount the stage volume. This specification remains the same for both the stage volume implementations.

Guidelines when using stage volumes¶

This section provides you with guidelines to follow when you implement application code in which a container uses a Snowflake stage as storage volume.

Common guidelines for both implementations of stage volumes¶

Stage mount is optimized for sequential reads and writes.
Stage mount I/O operations might have higher latencies than I/O operations on the container’s file system and block storage volumes. You should always check the status code of I/O operations to ensure they succeeded.
Stage mounts upload file updates asynchronously. Changes to files on a stage mount are only guaranteed to be persisted to the stage after the file descriptor is successfully closed or flushed. There might be a delay before the changes to files on a stage mount become visible to other containers and Snowflake.
Each directory in a mounted stage should contain fewer than 100,000 files. Expect readdir latency to increase with the number of files in the directory.

Guidelines when using the deprecated version of the stage volume implementation¶

Avoid concurrently writing to multiple files within a stage mount.
Stage mount isn’t a network file system. Don’t use stage mounts for multi-client coordination.
Don’t open multiple handles to the same file concurrently. Use opened file handles for either read or write operations, but not a mixture of both. To read from a file after writing to it, close the file and then re-open the file before reading.

Guidelines when using the generally available stage volume implementation¶

Concurrent writes to the same file from multiple stage mounts — same stage volume mounted on different containers — aren’t recommended.
The absence of a local disk cache improves consistency across mounts. File changes are flushed directly to the backing stage upon closing the file, with no local disk buffering. Reads always return the latest data, making the new stage mount better for sharing data between services.
Read and write data in large, contiguous chunks for optimal performance. The performance penalty for small reads and writes when compared to the generally available stage volume implementation, can mitigate the performance gains from the new implementation.

Limitations when using stage volumes¶

This section describes limitations you should be aware of when you implement application code in which containers use stage volumes. If you encounter any issues with these limits, contact your account representative.

Common limitations for both implementations of stage volumes¶

You can only mount a stage or a subdirectory in a stage; for example, @my_stage, @my_stage/folder. You can’t mount a single file in a stage; for example, @my_stage/folder/file.
External stages aren’t supported. Only Snowflake internal stages are supported.
A maximum of 5 stage volumes is allowed per service. For more information, see spec.volumes.
A maximum of 8 stage volumes per compute pool node are supported. Snowflake manages the stage mount per node limit similar to how it manages memory, CPU, and GPU. Launching a new service instance can cause Snowflake to launch new nodes when no existing nodes have the capacity to support the requested stage mounts.
Stage mounts are not fully POSIX compatible file systems. For example:
- File and directory renames are not atomic.
- Hard links are not supported.
The Linux kernel subsystem inode notify (inotify) that monitors changes to file systems doesn’t work on stage mounts.

Limitations when using the deprecated version of the stage volume implementation¶

The stage volume capabilities vary depending on the cloud platform for your Snowflake account:
- Accounts on AWS support internal stages with both SNOWFLAKE_FULL and SNOWFLAKE_SSE stage encryption. For more information, see Internal stage parameters.
- Accounts on Azure currently support internal stages with SNOWFLAKE_SSE encryption. When you run CREATE STAGE, use the ENCRYPTION parameter to specify the encryption type: CREATE STAGE my_stage ENCRYPTION = (TYPE = 'SNOWFLAKE_SSE');
- Accounts on Google Cloud aren’t supported.
Concurrent writes to the same file from multiple stage mounts — that is, the same stage volume mounted on different containers — aren’t supported.

Limitations when using the generally available version of the stage volume implementation¶

Random writes, and file appends aren’t supported.
Each stage that is mounted requires 512 MB memory per stage. This means that there is a limitation on the number of stage volumes that can be used based on instance size. Mounting the volume on multiple containers doesn’t increase memory consumption.