2.3.1 Log Entry

A log entry is a sequence of 4-KB sectors, aligned to a 4-KB sector.<3> This sequence is divided into two parts: a sequence of one or more descriptor sectors, and a sequence of zero or more data sectors.

The descriptor sectors contain header information and describe the writes that are being performed through the log, while the data sectors contain the actual updates. The first descriptor sector contains a 64-byte header and can contain up to 126 32-byte descriptors; any subsequent descriptor sectors that are part of the same log entry do not contain a header and could contain up to 128 32-byte descriptors. The total number of descriptors contained in the log entry is described in the log entry header. Each descriptor could be either a data descriptor or a zero descriptor. Each of the data descriptors describes a 4-KB sector write to perform on the file, and MUST be associated with a data sector that contains the actual 4-KB update to be written. The number of data sectors in the log entry must be equal to the number of data descriptors contained in that log entry. The zero descriptor describes a section of the file to be zero but does not need an associated sector to describe the update, because it is implied to be zero.

Figure 5: Log Entry Structure Layout Example

The preceding figure shows a log entry that contains four descriptors. The layout contains, in the first 4-KB descriptor sector, a log header with a descriptor count of 4 and associated descriptor sectors, three data descriptors, and one zero descriptor. It is followed by three data sectors, each of length 4 KB.

A variety of techniques is applied to validate the log entry. The entry header sequence number is duplicated in each descriptor and data sector. In the case of a data sector, the sequence number is split between the beginning and end of each data sector. Most cases of torn, missing, or misdirected writes will result in the sequence number validation being incorrect, either between the data descriptor and the associated data sector or between the descriptors and the entry header. Furthermore, a 32-bit CRC is computed over the entire log entry, including the sequence number; this improves the probability that a random entry corruption is detected.

The space for the log entries can be reused for subsequent log writes only when the log entries are not part of the active log sequence (See section 2.3.3).