IBM 000-8697 Sorted Write, Idle Write

2-76 IBM Informix OnLine Database Server Administrator’s Guide
Write Types Describe Flushing Activity
Data is always written to the primary chunk first. If a mirror chunk is
associatedwith the primary chunk, the write is repeated on the mirror chunk.
The write to the mirror chunk is also included in these counts.
Refer topage 5-17 for a discussion of tuning OnLine performance by
monitoring write-type statistics.
Referto page 7-87 for information about monitoringwrite types (and buffer
flushing) using tbstat -F.
Sorted Write
AnyOnLine process that is writing more than one page to the same disk sorts
the pages into disk sequence and writes the pages in disk-sequence order.
(The disk-sequence information is contained in the physical address of the
page,which is contained in the page header.) This technique is referred to as
asorted write.
Sorted writes are more efficient than either idle writes or foreground writes
because they minimize head movement (disk seek time) on the disk. In
addition, a sorted write enables the page cleaners to use the big buffers
duringthe write, if possible. (Refer to page 2-55 for moreinformation about
big buffers.)
Chunk writes, which occur during checkpoints, are performed as sorted
writes.(Chunk writes are the most efficient writes available to OnLine. Refer
topage 2-77.)
Idle Write
Writes that are initiated by the page cleaners are calledidle writes. The page-
cleaner daemon wakes periodically and searches through theMLRU queues
todetermine if the number of modified buffers is equal to or greater than the
value represented byLRU_MAX_DIRTY.
Ifa page cleaner determines that the buffer pool should be flushed, it marks
a page for flushing, flushes the page (after first checking to determine if the
physical log buffer must be flushed first), and then rechecks the percentage.
This process repeats until the number of modified buffers is less than the
value represented byLRU_MIN_DIRTY.