Windows® Internals, Sixth Edition, Part 2

Internally, the CLFS driver places the containers in a container queue to give clients a logical view of a single contiguous physical log stream; in doing so, the CLFS driver maps the physical container identifier to a logical container identifier. Containers are recycled when the tail of the active log migrates beyond the last sector of the container. Recycling a container involves moving it from the tail to the head of the container queue and appropriately updating its logical container identifier.

Log Sequence Numbers

When a client writes a record to a stream, CLFS returns a log sequence number (LSN) that identifies the log record for future reference. The LSNs assigned to the records that are written to a particular stream form an increasing sequence. That is, the LSN assigned to a record that is written to a stream is always greater than the LSN assigned to the previous record written to that same stream. Two critical LSNs that the base log file keeps track of are the log start LSN and the restart LSN, which, as described earlier, are stored in the BLF metadata.

An LSN is 64 bits wide and consists of three parts, as shown in Figure 12-15:

A 32-bit container index that identifies the log container where the log record resides

A 23-bit block offset that identifies an offset within a container

A 9-bit record offset that identifies a record within a block

Figure 12-15. CLFS LSN structure

Log Blocks

Because it is possible that a write to a log might fail, which is called a torn write, CLFS uses log blocks to track whether log records are fully committed to storage. CLFS stores log records within log blocks, which correspond to 512-byte sectors, and reads and writes data to a log using log blocks. Each log block includes a 2-byte sector signature at the end of each sector in the block that stores a sequence number and flags, as well as a copy of the most recently committed signatures in a signature array at the end of the block, as shown in Figure 12-16. Only if all the sector signatures in a log block are valid and match the signatures in the array, does CLFS consider the block valid. If a log block is partially written and a system failure occurs, for example, the signatures won’t match, and CLFS considers the log block invalid.

Figure 12-16. CLFS log blocks

Owner Pages

As mentioned previously, each 512-KB block of data in a multiplexed log (called a region) is correlated with its virtual log through an owner page. Each region consists of 4-KB pages, and each page contains one or more sectors, which contain log blocks. The owner page is the last page of a region, as shown in Figure 12-17. Because the owner page is itself a log block, CLFS can detect torn writes on the owner page, just as for a log record, by using the log block signature array.

Figure 12-17. CLFS regions and owner pages

An owner page contains two kinds of information:

For each sector in the region, the virtual log to which the sector belongs as well as the sector’s serial number (starting from 0). There can be at most 1,024 sectors in a region.

For each virtual log, the minimum and maximum virtual log LSN for the region. These values give the range of valid virtual LSNs for the region.

CLFS can tell by looking at the owner page of a virtual log LSN whether the record specified by the LSN resides in the current region or not. If the record does not reside in the current region, CLFS can decide whether it should search the previous region or the next region by comparing the virtual log LSN with the virtual log LSN range for the region.

When CLFS inserts log blocks into a multiplexed log’s physical FCB flush queue, if it finds that the current log block will overlap the owner page of the current region, it splits the current log block and inserts an owner page log block after the first half of the split log block (as shown in Figure 12-17). In other words, the owner page is written to disk only after the region that it describes becomes full. When a client reopens a multiplexed log file, CLFS scans the regions and rebuilds an in-memory owner page describing the latest region for which it hasn’t written an owner page log block.