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· Aug 29, 2017

pButtons: WDphase - Phase of the Write Daemon- 8: Databases are being updated. What does it mean?

#Monitoring #Performance #Red Hat Enterprise Linux (RHEL) #System Administration #Caché
Caché Version String: Cache for UNIX (Red Hat Enterprise Linux for x86-64) 2016.2.1

 

We have a mirrored Ensemble system (110,  backup and 210, primary). At one time (14:00) there is a disruption in the production. The messages are not being processed. 

Looking at the pButtons (every 10 seconds) I see the following abnormal at the WDphase

and the backup

The different values of WDphase are:

0: Idle (WD is not running)

5: WD is updating the Write Image Journal (WIJ) file.

7: WD is committing WIJ and Journal.

8: Databases are being updated.

 

Can anyone explain what the different phase of the write daemon mean, and what can be the cause that the backup site of the mirror keeps giving the WD phase 8.

PS

We have looked at the storage on server 110. We did not see any thing.

wdphase_110.jpg
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Sebastian Musielak · Sep 5, 2017

Hello Hans,

To expand on Fabian's comments, the Write Daemon goes into Phase 8 just before the process issues a disk write command to the Linux operating system.  The Write Daemon remains in phase 8 for the duration of the disk write and only moves on after the operating system returns a 'success' for the disk write.  

In your case, it seems that your Backup mirror member remains in phase 8 for a few more seconds than expected when compared to the primary member.  The most likely cause is that writes to disk are taking longer on your backup than your primary.  

You can confirm this by looking at the pButtons report.  The report has logs of Linux OS tools like 'SAR -d', 'VMSTAT', and 'IOSTAT', all of which should report IO statistics such as disk write time.  If your disk write time is high, then the WD will be in phase 8 for a longer time.  

I hope this helps.

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