CFT: TRIM Consolodation on UFS/FFS filesystems

[bob prohaska]

On Wed, Sep 05, 2018 at 03:07:16PM -0700, Kirk McKusick wrote:
> 
> It is a bit tricky to just turn off TRIM and then measure it, because
> in the immediate aftermath of its having been used, it will leave
> behind a legacy of easier to use flash areas yet not have the cost
> of keeping them cleaned up. So, it might initially look like enabling
> TRIM is a bad idea. Thus you would have to run many installworlds
> without TRIM enabled to see what the long-term result of not using
> TRIM turns out to be.
> 

Just for fun I ran a (somewhat absurd) -j4 buildworld on RPI3 using 6 GB of swap,
three on USB and three on microSD, just to see if anything interesting (bad) happened. 
The process took about 24 hours, and  the oversuppy of swap didn't cause any obvious 
problems. Next I turned on TRIM and re-ran the buildworld script.  

There were no obvious problems, but the process took about an extra hour.
Since /var, /tmp and /usr were all on USB there was no hope TRIM could be
any help on the busy filesystems. TRIM was enabled on microSD, but it had
little to do. There does seem to be a modest penalty for using TRIM when
it can't help much.

Is there any hope of implementing something like TRIM for USB on the Pi?
It appears that congestion on USB is a serious bottlneck from time to time
just for traffic with /tmp and /usr. Adding swap to the mix makes it worse.

Log files are at
http://www.zefox.net/~fbsd/rpi3/swaptests/r338342/3gbsd_3gbusb/
in case they're of interest.

Thanks for reading!

bob prohaska