Iodepth

Iodepth is a parameter used in storage I/O benchmarking and tuning that specifies the maximum number of outstanding I/O operations that can be in flight for a given thread, queue, or I/O context. It is commonly exposed by benchmarking tools such as fio and by storage controllers that expose queue depth settings. The value determines how aggressively the system issues I/O requests ahead of completion.

Iodepth works with the device’s own queue and the system’s I/O scheduler. A higher iodepth can improve throughput on fast storage by exploiting parallelism, but it can also increase latency due to queuing delays and contention for resources within the storage stack. If the depth exceeds the device’s or array’s capability, additional requests may queue, leading to diminishing returns or increased latency. Conversely, too small a depth can leave the device underutilized and limit IOPS.

Guidelines for choosing iodepth depend on workload and hardware. For hard disk drives, values in the low tens to a few dozens are common; for SATA SSDs, mid-range depths are typical; for PCIe/NVMe SSDs, higher values in the hundreds or thousands may be beneficial. In virtualized environments, the effective iodepth is constrained by the guest and host virtualization layers, so empirical testing is recommended.

In fio, iodepth is the number of outstanding I/O requests per thread (or per job, depending on version). It is often paired with bs (block size), rw (read/write pattern), and numjobs to model concurrency. The parameter interacts with direct I/O and caching settings and should be tested across workloads to avoid bias.

Example: fio --name=benchmark --ioengine=libaio --iodepth=64 --rw=randread --bs=4k --size=1G --numjobs=4 --direct=1.