Set the WiredTiger leaf_page_max to 128KB automatically when creating cold collection data tables (disaggregated=(storage_tier=cold)). Currently, cold collections use the WiredTiger default of 32KB, and the only way to set a different page size is via the user-facing configString parameter in createCollection. This change should be transparent so no user action required.

See 128KB decision document for full analysis and rationale.

The change should be in WiredTigerRecordStore::generateCreateString(). In generateCreateString(), after the existing prefix config and before extraCreateOptions is appended, detect if disaggregated=(storage_tier=cold) is present in extraCreateOptions and, if so, inject leaf_page_max=128KB into the prefix. Because extraCreateOptions is appended after, users can still override with an explicit leaf_page_max in their configString.

Rough pseudocode for the change in generateCreateString():

// In the prefix section (before extraCreateOptions):
ss << "type=file,";
ss << "memory_page_max=" << wtTableConfig.memoryPageMax << ",";
ss << "split_pct=90,";
ss << "leaf_value_max=64MB,"; 

// NEW: default leaf page size for cold collections
if (wtTableConfig.extraCreateOptions.find("storage_tier=cold") != std::string::npos) {
    ss << "leaf_page_max=128KB,";
}

ss << "checksum=on,";
// ... rest of prefix ...
ss << wtTableConfig.extraCreateOptions << ",";  // user config overrides prefix 

Alternatively, the detection could happen in _createRecordStore() where extraCreateOptions is assembled, setting a flag on WiredTigerTableConfig like bool isColdCollection that generateCreateString() checks.

Scope

• Data tables only. Cold collection index tables are unaffected.
• User override preserved. Because extraCreateOptions is appended after the prefix, an explicit leaf_page_max=X in the user's configString will override the 128KB default (WiredTiger last-value-wins).

Acceptance Criteria

• {{db.createCollection("x", {storageEngine: {wiredTiger: {configString: "disaggregated=(storage_tier=cold)"}

  }})}} creates a table with leaf_page_max=128KB without the user specifying it.

• Non-cold collections are unaffected (remain at WT default 32KB).
• Users can still override with an explicit leaf_page_max in configString.