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fix(io): make the posix backend return the data it reads - #16
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The posix backend always allocated its buffers with cudaMalloc. It read each file into a host buffer and only copied it to the GPU buffer with --copy-to-gpu. Without that option, the buffers fil_next() returned were never written. The Python binding also treated these GPU pointers as host memory. And on a machine without a GPU, posix could not read any file, because the allocation failed. Without --copy-to-gpu, allocate the buffers in host memory, aligned for O_DIRECT, and read files straight into them. The extra host buffer is then not needed. cuFile is unchanged, as it always reads into GPU memory. Assisted-by: Claude Code:claude-opus-5 Signed-off-by: Karl Bonde Torp <k.torp@samsung.com>
When read() returns fewer bytes than asked for, the posix backend reads the rest in a loop. Each call wrote to the start of the buffer, so the rest of the file overwrote the part already read. Read into the buffer after the bytes already read. Assisted-by: Claude Code:claude-opus-5-5 Signed-off-by: Karl Bonde Torp <k.torp@samsung.com>
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Two fixes to the posix backend. Both are needed for it to return correct data without --copy-to-gpu.
This is pre-work for the upcoming CI PR: #14