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Hauser
48 lines
2.5 KiB
Markdown
48 lines
2.5 KiB
Markdown
# io_uring Read Chunk Size
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**Use this when:** tuning the maximum logical size of one io_uring read operation without changing descriptor caching, shard count or queue depth.
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**Source of truth:** `crates/ecstore/src/disk/local.rs` (`UringBackend`, `pread_uring`, `pread_uring_direct`) and `crates/ecstore/src/disk/uring_read_chunks.rs` (`ReadChunkSize`).
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## Configuration
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`RUSTFS_IO_URING_READ_CHUNK_BYTES` is an unsigned decimal byte count from `4096`
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through `134217728` (128 MiB), inclusive. When unset, the existing 128 MiB cap
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is retained. Empty, signed, whitespace-padded, non-Unicode, zero, overflowing
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and out-of-range values are rejected: io_uring backend construction falls back
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to std with one warning that does not print the raw value.
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The setting is read once on first io_uring construction and requires a process
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restart to change. It does not enable io_uring by itself. Disabled io_uring and
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non-Linux production paths continue to use the standard backend.
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## Read behavior
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Each backend retains a validated cap. Both buffered and direct positioned reads
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use one driver operation for a logical range no larger than that cap; larger
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ranges use sequential chunks and assemble the complete result. Zero-length
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reads preserve their existing validation and result semantics.
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The cap need not be a power of two or a device-block multiple: `4097` is valid.
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For direct reads, the existing driver aligns each chunk's enclosing physical
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range and returns only its logical bytes. Adjacent operations can reread boundary
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blocks. Length/EOF checks, error classification, fallback and page-cache reclaim
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remain in their existing paths; a short chunk is not accepted as a complete
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object-range result.
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Smaller chunks trade lower logical bytes per submitted operation for more
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submissions, wakeups and result assembly. The default is unchanged and there is
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no measured performance claim. Compare matched workloads before adopting a
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smaller deployment value.
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## Not a byte quota
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This is a per-operation logical length cap, not a weighted admission budget.
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Direct alignment head/tail and allocation padding can exceed it. Concurrent
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operations across rings and disks multiply allocation, and canceled operations
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may remain owned by the driver until completion.
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The full assembly allocation, completed/retained results, allocator overhead,
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kernel resources and std fallback allocations are outside this cap. Changing
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chunk size therefore does not enforce process RSS, total request memory or an
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end-to-end read-buffer quota, and does not replace a driver-owned byte lease.
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