About Preallocating Journal Space

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As suggested earlier, for best performance journal files should be placed on a dedicated hard drive. If they are not, access to other files on the drive causes the disk heads to move away from the journal files, which forces a seek-back to the journal file before a new page is written. In addition, each time the journal file size increases, even when the journal files are on a dedicated drive, the disk heads seek away from the journal file so the operating system can update the directory information. To allow the disk heads to remain on the current journal file at all times, you can use the PREALLOCATE clause. The PREALLOCATE clause enables you to allocate space equal to the maximum number of journal files that will exist during normal operation, multiplied by the length of the journal files specified in the LENGTH clause of the CREATE JOURNAL statement.

If the journal is not on a dedicated drive, you can use the PREALLOCATE clause to allocate space equal to the size of the maximum number of journal files that might exist. This guarantees that other files cannot consume the space that may be needed for the journal. If journal archiving is enabled, and you are archiving to a remote machine, allocate enough space to accommodate the journal files that will accumulate if the connection to the remove machine is lost and the journal files cannot be archived for a period of time.