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I created a persistent Debian 9 live usb. The persistence is configured with / union. An unexpected consequence, although obvious in hindsight, is the system lags on non-cached reads:

holmes@bakerst:~$ # WRITE to disk
holmes@bakerst:~$ dd if=/dev/zero of=tempfile bs=1M count=1024; sync
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.417477 s, 2.6 GB/s

holmes@bakerst:~$ # READ from buffer
holmes@bakerst:~$ dd if=tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.0907808 s, 11.8 GB/s

holmes@bakerst:~$ # Clear cache, non-cached READ speed
holmes@bakerst:~$ sudo /sbin/sysctl -w vm.drop_caches=3
vm.drop_caches = 3
holmes@bakerst:~$ dd if=tempfile of=/dev/null bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 15.3935 s, 69.8 MB/s

There is a 169X difference between cached and non-cached read operations! What can I do, if anything, to improve performance?

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  • Get a faster USB 3 pendrive, or maybe even a USB SSD :-)

  • You can easily improve reading from the image of the iso file (after a slow start), put all the content of the squash file system into RAM with the boot option toram, but I don't think it is easy or meaningful to do that with the content of the file/partition for persistence. See this link for more details.

The following screenshot of the grub menu of a persistent live system made by mkusb is from Ubuntu, but looks very similar for Debian. There is already a menuentry for toram.

enter image description here

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  • Running from USB-C reads twice as fast (~111 Mb/s vs ~69 Mb/s) for non-cached. This seems sufficient to perceive the system as "smooth". The other speeds remain about the same. Unfortunately, the flash drive runs hot but I suppose that's a subject for another question... :)
    – Lorem Ipsum
    Feb 5, 2019 at 17:20
  • @LoremIpsum, Yes, that's a subject for another question. Some of my flash drives run hot, some don't. Typically fast drives run hotter than slow drives, and new drives run hotter than old ones. I don't know enough about the hardware to tell if it is a problem (for example at what temperature the lifetime is reduced.)
    – sudodus
    Feb 5, 2019 at 19:38

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