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## I/O Tester utility
The I/O Tester utility, `io_tester` generates a user-defined I/O pattern
spanning one of multiple shards that is designed to simulate the I/O behavior
of a complex Seastar application.
# Running I/O tester:
I/O tester takes the same options as Seastar, and those options may be used
to test the behavior of I/O under the circumnstances established by those
options. For instance, one may adjust the `--task-quota-ms` option to see
if that affects higher percentile latencies.
Aside from the usual seastar options, I/O tester accepts the following options:
* `duration`: for how long to run the evaluation,
* `directory`: a directory where to run the evaluation (it must be on XFS),
* `conf`: the path to a YAML file describing the evaluation.
# Describing the evaluation
The evaluation is described in a YAML file that contains multiple classes.
Each class spans jobs of similar characteristics in different shards and (for now)
all jobs run concurrently.
The YAML file contains a list of maps where each element of the list describes a class.
A class has some properties that are common to all elements of the class, and a nested map
that contain properties of a job (class instance in a shard)
For example:
```
- name: big_writes
type: seqread
shards: all
shard_info:
parallelism: 10
reqsize: 256kB
shares: 10
think_time: 0
```
* `name`: mandatory property, a string that identifies jobs of this class
* `type`: mandatory property, one of seqread, seqwrite, randread, randwrite, append, cpu
* `shards`: mandatory property, either the string "all" or a list of shards where this class should place jobs.
The properties under `shard_info` represent properties of the job that will
be replicated to each shard. All properties under `shard_info` are optional, and in case not specified, defaults are used.
* `parallelism`: the amount of parallel requests this job will generate in a specific shard. Requests can be either active or thinking (see `think_time`)
* `reqsize` : (I/O loads only) the size of requests generated by this job
* `shares` : how many shares requests in this job will have in the scheduler
* `think_time`: how long to wait before submitting another request in this job once one finishes.
* `execution_time`: (cpu loads only) for how long to execute a CPU loop
# Example output
```
Creating initial files...
Starting evaluation...
Shard 0
Class 0(big_writes: 10 shares, 262144-byte SEQ WRITE, 10 concurrent requests, NO think time)
Throughput : 436556 KB/s
Lat average : 5847 usec
Lat quantile= 0.5 : 2678 usec
Lat quantile= 0.95 : 13029 usec
Lat quantile= 0.99 : 20835 usec
Lat quantile=0.999 : 246090 usec
Lat max : 450785 usec
```
# Future
Some ideas for extending I/O tester:
* allow properties like think time, request size, etc, to be specified as distributions instead of a fixed number
* allow classes to have class-wide properties. For instance, we could define a class with parallelism of 100, and distribute those 100 requests over all shards in which this class is placed
* allow some jobs to be executed sequentially in relationship to others, so we can have preparation jobs.
* support other types, like delete, fsync, etc.
* provide functionality similar to diskplorer.
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