Read, write to USB disk cache

[Toutatis]

Hello,
Could you tell me how to purge, clear, the USB disk read/write cache with command line on ssh session.
Thanks

[eduperez]

If you are going to extract the drive, the "mount" command will not finish untill all pending writes have finished.

Otherwise, there exists the "fsync" system call, and the "sync" command.

[Toutatis]

Not exactly my purpose;
After write, I want to purge the cache to measure the read time with disk access.
I don't think the sync/fsync will purge the RAM cache I think it will copy the data in the disk.

[eduperez]

Yes, fsync will force pending writes to the drive, but it will also retain the cache in RAM. I do not know of a specific command to delete the cache, but unmounting the drive will surely have that effect.

[Toutatis]

I found it here
It's working for Openwrt

Many thanks

Edit

free -m
              total        used        free      shared  buff/cache   available
Mem:         511968      185620       28996         332      297352      285400
Swap:             0           0           0
~# sync; echo 3 > /proc/sys/vm/drop_caches
~# free -m
              total        used        free      shared  buff/cache   available
Mem:         511968      182460      323880         332        5628      304768
Swap:             0           0           0

[tmomas]

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