I'm using an opi PC with a sandisk ultra 32GB micro SD. I've noticed that if you do a large write to the SD card (i.e. via samba) then the system becomes sluggish. Writing to a USB stick does not have the same effect on system performance.

 

I tried changing the scheduler to CFQ (from noop) and it helped a lot. Has there been any testing on which scheduler is best? I assume noop performance will vary dependant on the SD controller?

 

raspbian uses deadline as the default - https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=64702

 

Some very good info here:

https://www.thomas-krenn.com/de/wiki/Linux_I/O_Scheduler

 

Scheduler for SSDs

The Deadline Scheduler is well-suited for interacting with SSDs because it does not rely on the excessive merging of requests. In many places, NOOP is praised as the best scheduler for SSDs since it simply passes the requests to the device. For performance reasons this is quite conclusive, but the fairness of the scheduler times under applications can suffer.

 

Why not solving the problem instead and choosing storage media that doesn't suck? There's a pinned thread in Free forum regarding random I/O performance of various SD cards (TL;DR: buy Samsung EVO with 32 or 64 GB if you feel you want to use SD cards for 'real storage')