damn slow SDHC card

[Peter Warasin <peter@endian.com>]

hi guys

i have /var and swap on an SD card (sandisk class4 4GB) in an embedded
network device, which turns out to be reeeeally slow. System under
certain circumstances freezes and in top i see iowait at 90% and
processes which touch sd card, like kswapd, kjournald and *mmc*d are all
on io-wait (D)

i found some help at linaro for sorting out if it is the SD card, which
most probably is and try now to search a card which is fast enough

but tried also to sort out if maybe it is the kernel, because something
is fishy there.
can you guys help me a little on that?

the card is a high-speed card, but it is not running in highspeed mode:

cat /sys/kernel/debug/mmc0/ios
clock:		25000000 Hz
vdd:		20 (3.2 ~ 3.3 V)
bus mode:	2 (push-pull)
chip select:	0 (don't care)
power mode:	2 (on)
bus width:	2 (4 bits)
timing spec:	0 (legacy)


i then forced the card setting to highspeed mode (by commenting out some
returns in mmc_switch_hs() and found out that the card meets all
requirements for highspeed, but

card->sw_caps.hs_max_dtr is 0

is that value the maximum bus speed which the card can handle?
i assume when it is 0 it means it is limited to legacy specifications,
which is quite slow (?)

or is it regardless for performance if a SDHC card runs in legacy mode?

thank you in advance

kind regards,
peter


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