What is Write ahead log

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An efficient database algorithm. For the same amount of data, while using WAL log, database system during the transaction commits disk writes only about half of the traditional rollback log, greatly improving the efficiency of the database disk I/O operations, thereby improving the performance of the database. Each incremental data loads in HBase are written to WAL.

Other related questions:
Write penalty
RAID Write Penalty Write penalties for common RAID levels are as follows: RAID 0 (stripe): Data is written to physical disks once. The RAID 0 write penalty value is 1. RAID 1 (mirror) and RAID 10: The write penalty of RAID 1 and RAID 10 is 2, because data mirroring results in two data write operations in each data write. RAID 5: Because the parity bit calculation mechanism exists in RAID 5, data reading, parity bit reading, data writing, and parity bit writing are performed. Therefore, the write penalty value of RAID 5 is 4. RAID 6: RAID 6 has two parity bits. The parity bits need to be read and written twice. Therefore, the write penalty value of RAID 6 is 6.

How does the S12700 write logs to eUSB card
The logs are cached instead of being immediately written into the log files on the eUSB card. The S12700 supports two methods to write the logs: On the S12700, the cache can store 33 KB logs. When the cache is full, the logs are written into the log file on the eUSB card. You can also store logs to the eUSB card manually. That is, run the save logfile command in the user view. The information in the cache can be immediately written into the log file on the eUSB card. NOTE: Before providing the logs to R&D, import the logs into log files.

How does the S7700&S9700 write logs to CF card
The logs are cached instead of being immediately written into the log files on the CF card. The S7700&S9700 supports two methods to write the logs: On the S7700&S9700, the cache can store 33 KB logs. When the cache is full, the logs are written into the log file on the CF card. You can also store logs to the CF card manually. That is, run the save logfile command in the user view. The information in the cache can be immediately written into the log file on the CF card. NOTE: Before providing the logs to R&D, import the logs into log files.

Feature of the LUN write through policy
1. After data is written to the controller, the controller needs to write the data to the disk and then feed back the acknowledgement information to the upper-layer host. 2. The reliability of data written through is increased and the performance is low.

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