Grafana Mimir binary index-header

To query series inside blocks from object storage, the store-gateway must obtain information about each block index. To obtain the required information, the store-gateway builds an index-header for each block and stores it on local disk.

The store-gateway uses GET byte range request to build the index-header, which contains specific sections of the block’s index. The store-gateway uses the index-header at query time.

Because downloading specific sections of the original block’s index is a computationally easy operation, the index-header is not uploaded to the object storage. If the index-header is not available on local disk, store-gateway instances (or the same instance after a rolling update completes without a persistent disk) re-build the index-header from the original block’s index.

Format (version 1)

The index-header is a subset of the block index and contains:

Symbol Table: Used to unintern string values
Posting Offset Table: Used to look up postings

The following example shows the format of the index-header file that is located in each block’s store-gateway local directory. It is terminated by a table of contents that serves as an entry point into the index.

┌─────────────────────────────┬───────────────────────────────┐
│    magic(0xBAAAD792) <4b>   │      version(1) <1 byte>      │
├─────────────────────────────┬───────────────────────────────┤
│  index version(2) <1 byte>  │ index PostingOffsetTable <8b> │
├─────────────────────────────┴───────────────────────────────┤
│ ┌─────────────────────────────────────────────────────────┐ │
│ │      Symbol Table (exact copy from original index)      │ │
│ ├─────────────────────────────────────────────────────────┤ │
│ │      Posting Offset Table (exact copy from index)       │ │
│ ├─────────────────────────────────────────────────────────┤ │
│ │                          TOC                            │ │
│ └─────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────┘