PV buffers and the volume group
AnthonyEnglish 270000RKFN Visits (12372)
I've been playing with LVM tunables, specifically to do with pbufs, to see if changes to the parameters stay with a volume group when it gets moved to a new LPAR.
First, some background
A pbuf is a pinned memory buffer. As this developerWorks article explains, "T
The lvmo command is used to manage pbuf tuning parameters. It allows you to view or set the pbuf count by volume group rather than doing it globally. You can see the number of blocked I/Os for a volume group using the lvmo command. You can also see the global blocked count (total for all volume groups) using vmstat -v, identified as pending disk I/Os blocked with no pbuf.
For now, the question is about setting the pv_pbuf_count.
I was curious. If the pv_pbuf_count (a pbuf count for each physical volume) is set on a volume group basis, where does it get stored? Is it in
Setting the pbuf count
First, I'll change a volume group's pv_pbuf_count from the default value of 512 to 2048 using lvmo:
lvmo -v datavg -o pv_pbuf_count=2048
Now' display the current settings and statistics using
lvmo -v datavg -a
vgname = datavgThe new pv_pbuf_count is set to 2048. We're allowed 2048 pbufs for each PV in the volume group. The total pbufs for the volume group (total_vg_pbufs) are also 2048. This is because the volume group only has one PV in it. The glob
This tunable parameter (pv_pbuf_count) survives a reboot, so where is this parameter change recorded? In /etc
So is the setting in the VGDA? I'll export the volume group using exportvg and import it and see what happens.
Ordinarily, you'd be doing the export from one LPAR, map the LUN to another LPAR and then import the volume group there, but doing the export and import on the same LPAR will prove the point for this exercise.
Before exporting the volume group, I need to unmount any file systems in it. You can list file systems in a volume group using the lsvgfs command. Having done that, you can deactivate and export the volume group:
And then import it again, to see what happens to our beloved pv_pbuf_count parameter.
Now, let's see what happened to the pv_pbuf_count:
lvmo -v datavg -a
Aha! The export and import has reset the pv_pbuf_count back to the default of 512.
When you do a volume group export and import - a great way of moving all of a volume group's data to a new location, rather than copying it or restoring it - the logical volumes and file systems get moved across to the target system, but tuning parameters don't come for the ride.