Tracing the storage stack

This document describes the storage stack‘s integration with Fuchsia’s tracing system system, and how you can use tracing to investigate performance, interactions, and behaviour in the storage stack.

For general background on how to use tracing, see Recording a trace.

Quick Start

Run the following command to take a trace for investigating general storage issues:

ffx trace start --categories "storage,blobfs,kernel,kernel:sched,minfs,fxfs" --duration 10

If you want to see detailed flows for page faults and user pager events, add the kernel:vm category as well:

ffx trace start --categories "storage,blobfs,kernel,kernel:sched,kernel:vm,minfs,fxfs" --duration 10

Once the trace is complete, an .fxt file will be generated. Load this file in to get started.

Storage Tracing

Fuchsia's tracing system mainly deals with [durations]{trace-durations} and [flows]{trace-flows}. We can see both of these in use in the following example, where durations are the blocks of execution and flows are the arrows pointing between durations in different threads or processes:

Example trace

Trace durations

Trace durations are the most common type of instrumentation. They record how long a given operation takes (measured both by wall time and CPU time).

Each filesystem (blobfs, minfs, and fxfs) has instrumentation for a variety of common filesystem operations. For example, in blobfs, every block transaction is instrumented to record the duration ( source):

  zx_status_t Transaction(block_fifo_request_t* requests, size_t count) final {
    TRACE_DURATION("blobfs", "Blobfs::Transaction", "count", count);
    return block_device_->FifoTransaction(requests, count);

Adding a new trace duration is simple. The TRACE_DURATION macro starts a new duration which ends when the current scope ends. Durations can take zero or more named paramenters, such as count in the example above. There is no performance overhead for trace durations when tracing is disabled, so feel free to use these liberally.

Trace flows

Flow events allow us to connect durations across threads or processes. This is useful for tracking asynchronous work. Currently, the block stack has basic support for trace flow events. Block operations which are performed through libtransaction will automatically be annotated with trace flow IDs. When interacting directly with the block FIFO, trace flow IDs must be manually assigned (see trace.h for details about how to generate appropriate IDs).

When adding trace flows, all of the flow events must be contained within a TRACE_DURATION, and the category and name of each of the flow events must match up with the other flows. For example, the block request flow events are annotated as such:

// The start of a block transaction. There must be exactly one of these per ID.
TRACE_FLOW_BEGIN("storage", "BlockOp", request.trace_flow_id);

// A step in a block transaction. We can have zero or more of these.
TRACE_FLOW_STEP("storage", "BlockOp", request.trace_flow_id);

// The end of a block transaction. There must be exactly one of these per ID.
TRACE_FLOW_END("storage", "BlockOp", request.trace_flow_id);

If we added another flow step, it must match the above definition exactly to be considered part of the overall flow. (The duration that encloses the flow step can be arbitrarily named, which is how we add more information about where we are in the flow.)

List of trace categories

The following categories are useful for storage work:

kernelKernel trace events. You almost always want this, since this provides the metadata for processes/threads.
kernel:schedKernel scheduler information. You almost always want this, as it provides thread state.
kernel:vmTrace events for page faults, which is useful for investigating fault performance. (This allows you to see the faulting thread.)
blobfsBlobfs-specific trace events.
minfsMinfs-specific trace events.
fxfsFxfs-specific trace events.
zxcryptzxcrypt-specific trace events.
storageGeneral storage trace events (block IO flows, for example)


There are only a few seconds worth of data in my trace

Most likely the trace buffer was too small. You can increase the trace buffer size with the --buffer-size flag (units are MB):

ffx trace start --categories "storage,blobfs,kernel,kernel:sched" --duration 5 --buffer-size 64