The log is a large linear byte space it is extremely convenient if the log is written -once as well as then read-only.
Space in the log is never re – written- This permits one to identify log records by the relative byte address of the last byte of the record.
A usual (small) transaction writes 500 bytes of log. One is able to run about one hundred such transactions per second on current hardware. There are around 100000 seconds in a day. Therefore the log can grow at 5 billion bytes per day. (More usually systems write four log tapes a day at 50 megabytes per tape.)
Given those statistics the log addresses must be about 48 bits long (good for 200 years on current hardware.)
Log manager should map this semi-infinite logical file (log) into the rather finite files (32 bit addresses) provided by the basic operating system. As one file is filled another is allocated as well as the old one is archived. Log manager offers other resource managers with the operations WRITE_LOG: cause the identified log record to be written to the log. Formerly a log record is written. It can merely be read. It can’t be edited. WRITE_LOG is the basic command utilized by all resource managers to generate log records. It proceeds the address of the last byte of the written log record.
FORCE-LOG- causes the identified log record as well as all prior log records to be recorded in non-volatile storage. When it proceeds the writes have completed.
OPEN-LOG- point out that the issuer wishes to read the log of some transaction or read the whole log in sequential order. It produces a read cursor on the log.
SEARCH-LOG- moves the cursor a designated number of bytes or else until a log record satisfying few criterion is located.
READ-LOG- Requests that the log record currently selected through the log cursor be read.
CHECK-LOG- Permits the issuer to test whether a record has been placed in the non-volatile log as well as optionally to wait until the log record has been written out.
GET-CURSOR- causes the recent value of the write cursor to be returned to the issuer. The RBA (relative byte address) returned perhaps used at a later time to position a read cursor.
CLOSE-LOG: Point to the issuer is finished reading the log.
The write log operation moves a new log record to the finish of the current log buffer. If the buffer fills another is allocated as well as the write continues into the new buffer.
When a log buffer fills or else when a synchronous log write is issued a log daemon writes the buffer to non-volatile storage. Usually logs have been recorded on magnetic tape for the reason that it is so inexpensive to store as well as because the transfer rate is quite high. In the future disk CCD (non-volatile?) or magnetic bubbles possibly attractive as a staging device for the log. This is especially true for the reason that an on-line version of the log is very desirable for transaction undo and for fast restart.
It is significant to doubly record the log. If the log isn’t doubly recorded then a media error on the log device will produce a cold start of the system. The double log devices should be on separate paths so that if one device or path fails the system can continue in degraded mode (this is merely appropriate for applications requiring high availability.)
The subsequent problem is left as an exercise for the reader: We have decided to log to dedicated double disk drives. When a drive fills it will be recorded to a mass storage device. This archive procedure makes the disk unavailable to the log manager (because of arm contention.) Explain a scheme which:
• Minimizes the amount of drives required, .• For all time it has a large disk reserve of free disk space, and• For all time it has a large fraction of the recent section of the log on line.
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