Background

__wti_rec_pack_delta_row_leaf is called once per changed key during disaggregated leaf delta reconciliation. It encodes a flags byte and the cell value into a custom buffer using the format WT_DELTA_LEAF_VALUE_FORMAT = "Bu".

Problem

The encoding used _wt_struct_size followed by _wt_struct_pack — two general-purpose packing calls that each set up a va_list, iterate the format string, and call __pack_next per field. This overhead was paid on every key in the delta, making it a hot-path cost in write-heavy workloads.

Fix

"Bu" encodes exactly one raw byte followed by raw item data with no length prefix, so the encoded size is always 1 + value.size. Replace the two struct pack calls with a direct byte store of the flags and a memcpy of the value data.

custom_value_size = 1 + value.size;
WT_ERR(__wt_scr_alloc(session, custom_value_size, &custom_value));
((uint8_t *)custom_value->mem)[0] = flags;
if (value.size > 0)
    memcpy((uint8_t *)custom_value->mem + 1, value.data, value.size);
custom_value->size = custom_value_size;

This eliminates 2 function calls, 2 va_list setups, and 4 __pack_next iterations per key. The encoding produced is identical to what __wt_struct_pack would produce for this fixed format.