Communicator e3000 MPE/iX Release 6.5 (Non-Platform Software Release C.65.00 (30216-90291)

Chapter 5 131
Internet and Interoperability
XM User Checkpoint Priority Control
XM User Checkpoint Priority Control
Jyothi BS
Commercial Systems Division
Background
Whenever one half of the Transaction Manager (XM) user log gets full, XM starts the
checkpoint process to write out all the dirty pages of the files which have transactions
logged in that half.
During user Checkpoint on many busy high-end systems, the response to on-line users is
very poor. Sometimes, it almost stalls. The main reason is that there is too much of disk i/o
being used at very high priority, i.e., C152. This problem may increase further if the
userlog is expanded from the current 64 MB.
Solution Objective
The user should have the flexibility to easily alter the XM user chkpt priority (at user
level). The user-given priority value should be persistent across system boots.
Overview of the Solution
The goal of this solution is to be able to reduce the impact of XM user checkpoint operation
on the on-line response. This means controlling the priority at which chkpt i/o’s are issued
by MM, and also having the priority configurable by the user.
The priority is configurable at volume-set level and it is not system wide. The priority for
the required volume-set can be changed using volutil.
How and When to Control the Checkpoint Priority
The users should lower the priority only if they see a significant impact on-line response
time. It should not be made so low (such as 255) that checkpointing takes a long time and
system nearly stalls. It should not be so high (such as 152) that during checkpointing,
on-line response comes down drastically.
How to Give the Request to Change the Checkpoint Priority
The volutil tool is enhanced to provide two new commands to facilitate the control of
chkpt priority. They are,
SHOWCHKPTPRI <volset-name>
This command displays the priority of the checkpoint process for the given volume set.
ALTERCHKPTPRI <volset-name> <new priority>
This command modifies the priority of the User checkpoint process for the given volume
set. The checkpoint priority should be in the range of 152 to 255, with the recommended
value at 200. Priority 0 resets the checkpoint priority to default which is C152. The
changed priority becomes effective at the next checkpoint.