ici/sdr: optimize syscalls for file backed sdr - #107
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Every file-backed access in the SDR dataspace and transaction log used a separate lseek followed by read/write. The lseek syscall dominated the file-backed path, since scattered field updates seek on every call, and it left a window in which the file offset and the I/O could race. Convert all of them to single positioned pread/pwrite calls: - _sdrput dataspace write and _sdrfetch dataspace read (hot path) - _sdrput reversible undo-capture read of the pre-image - readFromLog log-entry read - reverseTransaction dataspace restore writes (rollback path) Reduction of syscalls yields better performance: 200k 64-byte writes, 16 per txn yield ~20-30% more throughput.
A reversible write logged its undo record in two steps -- a control header (offset, length) then the affected bytes' pre-image -- which for the persistent file log meant two write() syscalls per field updated. Replace writeToLog with writevToLog, which appends the whole entry from an iovec: for the file log a single positioned pwritev gathers the header and pre-image directly from their existing locations (no intermediate copy, one syscall instead of two).
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This yields ~20-30 % more throughput for file backed sdr