ScyllaDB chose asynchronous direct I/O (AIO/DIO) because it wanted the database—not the general-purpose Linux page cache—to control caching, read-ahead, request ordering, and I/O scheduling. Avi Kivity described that 2017 decision as choosing “the highest performing option, AIO/DIO,” a statement about ScyllaDB’s workload and design rather than a universal benchmark claim.
The choice also imposed costs: direct I/O requires careful alignment, and asynchronous completion makes application design more complex. Later ScyllaDB work discusses io_uring as a newer interface, but that is an evolution of the implementation context, not a reason to rewrite the original decision as if io_uring had been available in 2017.
The four Linux file-access paths
Linux applications commonly reach storage through four broad approaches. They differ mainly in who owns the cache, how data reaches the application, when a thread waits, and how much control the application has over scheduling.
| Method | Cache ownership | Data movement and CPU effects | Scheduling and waiting | Alignment and complexity |
|---|---|---|---|---|
Buffered read(2)/write(2) |
Kernel page cache | Copies data between kernel buffers and the process; ordinary alignment is handled automatically | Kernel decides much of request handling; calls can block | Simplest application model |
mmap |
Kernel page cache | Maps file pages into the process instead of using an explicit read copy; incurs more MMU activity | Page faults and kernel policy govern when data arrives | Convenient mapping model, but cache and scheduling remain outside application control |
| Direct I/O (DIO) | Application-managed; bypasses the page cache with O_DIRECT |
Less page-cache involvement and no normal buffered copy; buffers and offsets must satisfy alignment rules | The calling thread may block while the operation runs | More control, but alignment and cache management are the application’s responsibility |
| Asynchronous direct I/O (AIO/DIO) | Application-managed; bypasses the page cache | Direct-I/O constraints remain, while submissions can remain outstanding | Submission and completion are separate, allowing other work while storage operations are in flight | Greatest scheduling control in the original comparison, with the highest implementation complexity |
Why page-cache methods were not enough for ScyllaDB
Buffered I/O and mmap are strong general-purpose defaults because Linux can share cached data across processes and make automatic readahead and writeback decisions. That policy is necessarily generic, however. The kernel does not know which database pages are likely to be queried again, which writes are compaction output, or how a commit-log write should compete with a user query.
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ScyllaDB’s design already included database-specific knowledge and its own cache. Leaving storage policy to the kernel could therefore duplicate caching and make unrelated work compete in ways the database could avoid. The 2017 explanation by Avi Kivity presents bypassing the page cache as a way to put those decisions in the database’s control.
What AIO/DIO lets the database control
Application-level read-ahead and write-behind
ScyllaDB’s compaction path knows that much of the data it scans will be cold from a query perspective. The article describes using application-level read-ahead and write-behind while avoiding pollution of caches intended for application requests. This is a workload-specific policy, not a recommendation that every Linux program should bypass the page cache.
Query caching and read-ahead
For queries, the application can combine its knowledge of access patterns with its own cache and decide when to fetch additional data. That avoids treating every storage read as an interchangeable request managed by a system-wide policy.
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Predictable alignment
The ScyllaDB article describes aligning small reads to a 512-byte boundary. Direct I/O generally requires aligned buffers, offsets, and lengths appropriate to the filesystem and storage stack; the exact constraints must be validated for the target Linux configuration. Meeting them is the price of bypassing buffered-I/O convenience.
Bandwidth classes for competing work
ScyllaDB assigns I/O scheduling classes so queries, compaction, and commit-log writes can receive different treatment. This lets latency-sensitive foreground work coexist with background maintenance instead of relying solely on a kernel scheduler that lacks the database’s priorities.
Why asynchronous submission matters
With synchronous direct I/O, a thread can still stop and wait for each request. Asynchronous direct I/O separates submission from completion: the application submits work, continues useful execution, and later handles completion events. That separation allows a framework to keep storage queues full while assigning CPU time and I/O bandwidth according to database policy.
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The trade-off is software complexity. The application must manage request lifetime, aligned buffers, outstanding operations, completion handling, cancellation or failure paths, and backpressure. ScyllaDB uses Seastar to abstract these mechanics and expose callback and coroutine programming styles, according to Kivity’s 2017 article.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What “highest performing” means in ScyllaDB’s statement
Kivity wrote in the October 5, 2017 ScyllaDB article: “With ScyllaDB, we have chosen the highest performing option, AIO/DIO.” Read in context, this is ScyllaDB’s architectural choice for its latency-sensitive database workload, where application-controlled caching and scheduling are central. It is not evidence that AIO/DIO is fastest for every Linux application, filesystem, storage device, or access pattern.
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The available source set does not establish a single general performance percentage or a universal ranking. The meaningful comparison is therefore control versus convenience: buffered I/O and mmap reduce application work, while DIO and AIO/DIO transfer more policy responsibility to the application.
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The later io_uring context
Why io_uring entered the discussion
A ScyllaDB database-internals excerpt published November 25, 2024 describes shortcomings encountered with legacy Linux AIO and presents io_uring as a newer interface with a more convenient API. This later interface addresses how asynchronous operations are submitted and completed; it does not change the original reason ScyllaDB wanted direct, application-controlled I/O.
Seastar’s asymmetric backend work
In an engineering article dated July 22, 2026, ScyllaDB described an asymmetric io_uring backend for Seastar alongside the existing linux-aio backend. The stated aim was to offload work from application cores. That report said purely I/O-bound benchmarks showed no speedup, while the core backend had been merged into the official Seastar repository at the time of publication. Repository and release status can change, so those statements should be treated as dated engineering status rather than a permanent product guarantee.
The practical takeaway is that io_uring is a newer mechanism in the same broader effort: keep asynchronous I/O efficient while preserving application-level control. Choosing an interface should still be based on the workload, kernel and filesystem behavior, framework support, and the engineering cost of operating outside buffered I/O.
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Prefer buffered read/write when
- You want the simplest, most portable file-I/O code.
- Linux’s shared page cache and automatic readahead are useful rather than harmful.
- Your application does not have reliable knowledge to replace kernel cache policy.
Consider mmap when
- A memory-mapped programming model fits the access pattern.
- You accept page-fault-driven fetching and continued dependence on the kernel page cache.
- Reducing explicit read-call bookkeeping matters more than controlling I/O scheduling.
Consider direct I/O when
- Your application has its own cache and can prevent duplicate caching.
- You need to avoid page-cache pollution from streaming or background work.
- You can enforce the required buffer, offset, and length alignment and handle blocking explicitly.
Consider asynchronous direct I/O when
- You need many operations outstanding and want execution to continue while storage is busy.
- The application can prioritize classes of I/O and process completions safely.
- The performance and policy benefits justify substantially more implementation and operational complexity.
Bottom line for ScyllaDB
ScyllaDB selected AIO/DIO because its database engine could make better cache, read-ahead, and scheduling decisions than a generic page-cache policy for its workload. That decision required alignment discipline and a more elaborate asynchronous framework, but it matched ScyllaDB’s goal of coordinating query I/O with compaction and commit-log traffic. The later move to evaluate and implement io_uring concerns the interface used to deliver that control, not a claim that one API is universally fastest.
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