I found this in Teradata Database 13.10 Release Summary (B035-1098-109A) Page 18:
Allow Merging Data Blocks During Full File Operations
This features enables the Teradata file system to dynamically merge up to eight small data
blocks into a single, large data block per full-table modification scan. The feature:
• Adds a MERGEBLOCKRATIO option to the CREATE TABLE and ALTER TABLE
statements. The new option defines a limit on the size of the resulting block when multiple
existing blocks are being merged.
• Affects Filer: The TABLE /L command displays the current table level attribute for
MergeBlockRatio stored in the table header.
• Adds two tunable fields to the DBSControl utility:
• MERGEBLOCKRATIO defines a limit on the size of the resulting block when multiple
existing blocks are merged. It is applied to all existing databases in all tables across the
system where no specific table-level attribute exists.
• DISABLEMERGEBLOCKS disables all database merges across the system, causing all
modification operations on a table to ignore any MERGEBLOCKRATIO that would
otherwise apply to the table.
Having fewer, larger blocks reduces the I/O operations required to read from disk and modify
a large set of rows.
The feature only affects the performance of workloads that modify data. Read-only workloads
and insert operations into new subtables are not affected.
Check this link , it gives a detailed explanation for MERGEBLOCKRATIO :