LOB Chain Traversal

This guide explains how to inspect Large Object (LOB/BLOB) storage chains in InnoDB tablespaces using the --lob-chain flag on inno pages.

When to Use

Diagnosing LOB corruption — verify chain integrity when BLOB data reads fail
Storage analysis — understand how much space LOB data consumes across pages
Version investigation — examine LOB version chains for MVCC-related issues
Recovery assessment — determine which LOB pages are intact before attempting recovery

LOB Storage in InnoDB

InnoDB stores large values (TEXT, BLOB, JSON, long VARCHAR) externally when they exceed the inline threshold. There are two storage formats:

Old-Style BLOB Chains (Pre-8.0.12)

Simple linked list of BLOB pages (type FIL_PAGE_TYPE_BLOB):

INDEX page record ──▶ BLOB page 1 ──▶ BLOB page 2 ──▶ BLOB page 3
                      (first page)     (next page)     (next page)

New-Style LOB (MySQL 8.0.12+)

Three-level structure with index entries for partial updates:

INDEX page record ──▶ LOB_FIRST page
                      ├── LOB index entries (60 bytes each)
                      │   ├── entry 1 ──▶ LOB_DATA page
                      │   ├── entry 2 ──▶ LOB_DATA page
                      │   └── entry 3 ──▶ LOB_DATA page
                      └── LOB_INDEX pages (overflow index entries)

Compressed LOBs use ZLOB variants (ZLOB_FIRST, ZLOB_DATA, ZLOB_FRAG, ZLOB_FRAG_ENTRY, ZLOB_INDEX).

Using --lob-chain

# Show LOB chain info for all LOB pages
inno pages -f table.ibd --lob-chain

# Focus on a specific LOB first page
inno pages -f table.ibd -p 42 --lob-chain

# JSON output with chain details
inno pages -f table.ibd --lob-chain --json

When --lob-chain is enabled, LOB-type pages (Blob, ZBlob, LobFirst, LobData, LobIndex, ZlobFirst, ZlobData, ZlobIndex, ZlobFrag, ZlobFragEntry) display additional chain traversal information.

Chain Output

For each LOB chain origin (BLOB first page or LOB_FIRST page), the output includes:

Field	Description
Chain type	`old_blob`, `lob`, or `zlob`
First page	Page number of the chain start
Total data length	Sum of data across all pages in the chain
Page count	Number of pages in the chain
Pages	List of page numbers with per-page data lengths

Identifying LOB Pages

Use page type filtering to find LOB pages:

# List all LOB-related pages
inno pages -f table.ibd -t Blob
inno pages -f table.ibd -t LobFirst
inno pages -f table.ibd -t LobData

If a LOB chain is broken (missing page, corrupt pointer), the chain traversal will stop at the break point and report the last valid page. Compare the total_data_length against the expected column size to assess data loss.

inno pages -f table.ibd --lob-chain --json | jq '.[] | select(.lob_chain.page_count < 2)'

IDB Utils Documentation