The XEL method proposed in this article also performs log compression, but its continuous and incremental nature distinguishes it from previous log compression methods.
Section 2 provides a detailed explanation of how XEL works during both normal activity and recovery after a crash.
Both EL and XEL break the abstraction of the single FIFO queue that was presented in Section 1.
Another strong motivation for EL and XEL arises if a system can be augmented with a limited amount of nonvolatile main memory in which the first generation can reside.
Variations on EL and XEL can render a separate disk version of the database unnecessary: the most recently committed value for each object is always retained in the log.
EL and XEL maintain pointers to all records in the log that are relevant for recovery purposes.
XEL does not require timestamps for objects in the disk version of the database.
Figure 5 conveys the essence of XEL for the specific case of a log with three generations.
XEL performs physical state logging on the access path level, according to the taxonomy of Haerder and Reuter .
Apropos TLRs, XEL logs only commit events; it does not bother to log even the commit of a transaction that did not update any objects in the database.