To answer my own question here are my findings how to increase the speed of reading / writing to the SD card:
The tested card is a Sanddisk SDSDQAD-128G
The SD-card is formatted with EXT4.
I mainly have files >10MB on the SD-card.
To improve read performance it helps to increase the readahead buffer. If it is increased to 1024 KB it improves performance from approx 16Mbit/S to 44 Mbit/S
echo 1024 > /sys/devices/virtual/bdi/179\:0/read_ahead_kb
To Improve the write performance it helps to
- Disable journaling
- Set blocksize to 4096
- Optimise for largefile
mkfs.ext4 -O ^has_journal -b 4096 -T largefile /dev/mmcblk0p1
Change mount options
mount -t ext4 -O default,noatime,nodiratime,data=writeback /dev/mmcblk0p1 /home/SDmountpoint/
This improves write performance to Approx 40 Mbit/S