Write Cache without Deffer Write Topic is solved

FAQ, getting help, user experience about PrimoCache
Post Reply
cichy45
Level 4
Level 4
Posts: 38
Joined: Sun Oct 14, 2018 3:34 pm

Write Cache without Deffer Write

Post by cichy45 »

Hi, I have one question for support or more experienced users of PrimoCache. I have a small test setup with 300GB HDD and 60GB SSD as read cache. It is subjected to lots of small I/O so HDD can barely keep up (even being a VelociRaptor). I wonder if there is any performance benefit from enabling SSD as write cache without deffer write. As you can see from screenshot it is caching writes (Free Cache L2 Write capacity is decreasing indicating this) but I do not know how much (if at all) it helps increasing performance of this setup?
2019-02-17_125431.png
2019-02-17_125431.png (5.26 KiB) Viewed 2507 times
User avatar
Jaga
Contributor
Contributor
Posts: 692
Joined: Sat Jan 25, 2014 1:11 am

Re: Write Cache without Deffer Write

Post by Jaga »

It will absolutely help performance if you use a Read&Write strategy, even without deferred writes. The reason is simple - data being written to the hard drive with a read-only strategy doesn't hit the L2, but is instead dumped on the drive without any write congealing (it bypasses the L2 cache entirely). It won't be available as cached data until it is re-read back from the hard drive, which increases the drive's workload.

With the Read&Write strategy the data being written first hits the L2 cache, and then is congealed into larger writes (if possible) to the hard drive. When the system needs that data again it doesn't have to go to the drive since it is available in the L2 already. Workload on the HDD in this scenario is much less.
Post Reply