IBM 000-8697 Manual de usuario descargar pdf (Pagina 156)

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2-62 IBM Informix OnLine Database Server Administrator’s Guide

How a User Process Acquires a Buffer

Step 4: Read the Page in from Disk

If the requested page must be read from disk, the user process ﬁrst locates a

usable buffer in the FLRU queues. (Refer to page 2-57.) OnLine selects an

FLRU queue at random and tries to acquire the latch associated with the

queue.Ifthelatchcanbeacquired,thebufferatthe“least-recentlyused”end

of the queue is used. If another process holds the FLRU queue latch, the user

process tries to acquire a latch associated with another FLRU queue.

After a usable buffer is found, the buffer is temporarily removed from the

linked list that is the

FLRU queue. The user process acquires a latch on the

buffertablehashstructureandcreatesanentryinthe buffertableasthepage

is read from disk into the buffer.

Steps 5-7: Lock Buffer, Release Lock, and Wake Waiting Processes

Iftheuserprocessreadsthebufferwithout modifyingthedata,itreleasesthe

buffer as unmodiﬁed. If the user process had acquired the buffer with an

update or exclusive lock, other user processes may be waiting to read the

buffer.

The release of the buffer occurs in steps. First, the releasing user process

acquires a latch on the buffer table that enables it to modify the buffer entry.

Next, it looks to see if other user processes are sleeping or waiting for this

buffer. If so, the releasing user processwakes the ﬁrst processin the wait-list

queue that has a compatible lock-access type. The waiting processes are

queued according to priorities that encompass more than just “ﬁrst-come,

ﬁrst served” hierarchies. (Otherwise, user processes waiting for exclusive

access could wait forever.)

If no user process in the wait-list queue has a compatible lock-access type,

any user process waiting for that buffer can receive access.

Ifnoprocessiswaitingforthebuffer, thereleasingprocesstriesto releasethe

buffertotheFLRU queuewhereitwasfound. If the latch forthatFLRUqueue

is unavailable, the process tries to acquire a latch for a randomly selected

FLRU queue. When the FLRU queuelatch isacquired,the unmodiﬁedbuffer

is linked to the “most-recently used” end of the queue.

After thebuffer is returnedto theFLRU queue or the next user process in the

wait list is awakened, the releasing process removes itself from the user list

bit map for the buffer and decrements the shared-user count by one.

1 2 ... 151 152 153 154 155 156 157 158 159 160 161 ... 701 702

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