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May 30th, 2017 11:00
VMAX -SRDF/A Write Pacing - Help
Hello Community,
I am looking for some detailed information on SRDF/A Write Pacing, we have a situation where the cache utilization is high on the Remote Array (VMAX 20 K, running v 5876.288) and reaches WP limit very frequently causing the SRDF link to drop.
We have followed recommendations and have implemented DSE pool as well as rectified any link related problems which has improved the situation a lot.
However I would like to explore the Device Write Pacing option, and see if that can improve the cache utilization on the Remote Array. I did read the below content in the SRDF CLI user guide.
" Device-level pacing is for SRDF/A solutions in which the SRDF/A R2 devices participate in TimeFinder copy sessions.
SRDF/A device-level write pacing addresses conditions that lead to cache overflow specifically due to TimeFinder/Snap and TimeFinder/Clone sessions on an R2 device running in asynchronous mode."
All of our R2 devices have a Gold Copy associated with it in a precopy state, if I activate write pacing on the Gold Copy devices would it improve the cache utilization, also does it cause any performance impact on the hosts masked to it ?
nerdnair
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June 7th, 2017 11:00
Thanks Mark,
I did make some changes to the FAST policy on the Storage groups that has the R2 volumes to have the destage from cache to happen on faster disks, we have had less number of drops since then.
Is write pacing a good solution in my case or should be I thinking of adding more disks at this time ?
The source array is a VMAX400K and the remote array is a VMAX 20K .
Regards,
Vaishak
mark_rogov
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June 7th, 2017 11:00
Your R2 may be running out of back end disk capability. Check the heatmap. If the drives are overloaded, then destage from cache is slower, leading to overwhelming the WP. You may need to add drives to the R2 side.
JimK513
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June 8th, 2017 09:00
there are two types of write pacing. Device-level and Group-level.
group-level write pacing may apply. The documentation description is below but basically, group-level attempts to slow down host I/O rates so they don't overwhelm your rdf link capabilities. This may result in a slower response time to the applicable hosts.
We enabled both (device-level and group-level) in our specific environment, MSCS with SQL server with no noticeable affect in SQL IO performance.
It may help some but ultimately, we increased cache and link bandwidth to resolve our rdf issues.
Group-level write pacing
SRDF/A group-level write pacing detects when the I/O service rates are lower than the host I/O rates, and then takes corrective actions to slow down the host I/O rates to match the SRDF I/O service rates. When activated, this functionality performs write I/O pacing on all devices in the SRDF group during an SRDF/A session. This operation also provides an exemption capability to prevent group-level write I/O pacing on specified devices within a group.