TRIM on SD cards

[Bernd Walter]

It seems SD cards support a delete command, which FreeBSD supports
with the mmcsd driver.
newfs and tunefs support TRIM in that new filesystems are trim'ed
and the filesystems automatically trim free'ed blocks.
So far so good.
On the practical side with SD based ARM you don't write filesystems
directly via mmcsd.
We either create an image, which id dd'ed onto SD or in some cases
we use an USB SD drive.
With dd the unused blocks are written as well, which effectively
hurts by writing data.
Is there some kind of dd, which actually don't write zero blocks,
or even better does a trim call for them?
Is there a tool to trim a filesystem after creation?
Ok - even then there is the option to directly newfs on the SD card.
But in any case I need to be able to issue trim requests to the card.
It seems there is support in our da driver, but will it be transported
to the USB reader and even if - how can I find out if my reader actually
supports trim commands?
A newfs -t -E with my older transcend multireader and a new class10
Intenso micro-SD issues no error.
But is it save to assume the card actually got trim'ed?

Trim is enabled on the filesystem, but not shown in mount list:
[189]gw1# tunefs -t enable /dev/da0
tunefs: issue TRIM to the disk remains unchanged as enabled
[190]gw1# mount /dev/da0 /mnt
[191]gw1# mount | grep mnt
/dev/da0 on /mnt (ufs, local, soft-updates)
FreeBSD gw1.cicely.de 10.0-STABLE FreeBSD 10.0-STABLE #0: Thu Apr 17 11:47:45 CEST 2014     ticso at gw1.cicely.de:/usr/obj/home/builder/gw1/10/sys/GW1  i386

-- 
B.Walter <bernd at bwct.de> http://www.bwct.de
Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.