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Each log record in ARIES has a log sequence number (LSN) that uniquely identi es the record The number is conceptually just a logical identi er whose value is greater for log records that occur later in the log In practice, the LSN is generated in such a way that it can also be used to locate the log record on disk Typically, ARIES splits a log into multiple log les, each of which has a le number When a log le grows to some limit, ARIES appends further log records to a new log le; the new log le has a le number that is higher by 1 than the previous log le The LSN then consists of a le number and an offset within the le Each page also maintains an identi er called the PageLSN Whenever an operation (whether physical or logical) occurs on a page, the operation stores the LSN of its log record in the PageLSN eld of the page During the redo phase of recovery, any log records with LSN less than or equal to the PageLSN of a page should not be executed on the page, since their actions are already re ected on the page In combination with a scheme for recording PageLSNs as part of checkpointing, which we present later, ARIES can avoid even reading many pages for which logged operations are already re ected on disk Thereby recovery time is reduced signi cantly The PageLSN is essential for ensuring idempotence in the presence of physiological redo operations, since reapplying a physiological redo that has already been applied to a page could cause incorrect changes to a page Pages should not be ushed to disk while an update is in progress, since physiological operations cannot be redone on the partially updated state of the page on disk Therefore, ARIES uses latches on buffer pages to prevent them from being written to disk while they are being updated It releases the buffer page latch only after the update is completed, and the log record for the update has been written to the log Each log record also contains the LSN of the previous log record of the same transaction This value, stored in the PrevLSN eld, permits log records of a transaction to be fetched backward, without reading the whole log There are special redo-only log records generated during transaction rollback, called compensation log records (CLRs) in ARIES These serve the same purpose as the redo-only log records in our advanced recovery scheme In addition CLRs serve the role of the operation-abort log records in our scheme The CLRs have an extra eld, called the UndoNextLSN,.

The output of the command is fairly readable: there were four echo requests from the machine and four replies from the appliance (the last two sets were omitted from the output) To turn off the debug for ICMP, preface the preceding command with the no parameter: no debug icmp trace; or you could use the undebug all or no debug all commands

The McGraw Hill Companies, 2001

Starting in version 72, the security appliances support the traceroute command, which allows you to trace the layer 3 hops that packets go through to reach a destination Here is the syntax of the command:

Strategic alignment: As corporate objectives migrate regarding markets, products, revenue growth, and profit goals, sales management must continually update the sales department to support these changing business objectives Sometimes, major changes in strategy

17

ciscoasa# traceroute dst_ip_address [source src_ip_addr | logical_src_if_name] [numeric] [timeout timeout_value] [probe probe_num] [ttl min_ttl max_ttl] [port port_value] [use-icmp]

that records the LSN of the log that needs to be undone next, when the transaction is being rolled back This eld serves the same purpose as the operation identi er in the operation-abort log record in our scheme, which helps to skip over log records that have already been rolled back The DirtyPageTable contains a list of pages that have been updated in the database buffer For each page, it stores the PageLSN and a eld called the RecLSN which helps identify log records that have been applied already to the version of the page on disk When a page is inserted into the DirtyPageTable (when it is rst modi ed in the buffer pool) the value of RecLSN is set to the current end of log Whenever the page is ushed to disk, the page is removed from the DirtyPageTable A checkpoint log record contains the DirtyPageTable and a list of active transactions For each transaction, the checkpoint log record also notes LastLSN, the LSN of the last log record written by the transaction A xed position on disk also notes the LSN of the last (complete) checkpoint log record

The only required parameter is the destination IP address Optionally, you can specify a different source IP address on the appliance than the one it will use when exiting the destination interface Also, you can disable the reverse-DNS lookup with the numeric parameter The default timeout for replies is 3 seconds and can be changed with the timeout parameter The default number of probes for each layer 3 hop is 3, but can be changed with the probe parameter You can control the number of hops with