[Freedombox-discuss] apt-get update and an SD card

[Nick M. Daly]

On Thu, 17 May 2012 19:52:32 +0100, Nicholas Hardiman wrote:
> When I run apt-get upgrade, the top command tells me wa (time spent
> waiting for SD card IO) can sit at 95%. SD cards seem to have
> appalling write speeds.

Yup.

> Are you using any trick to make big write jobs less awful?

Some cards come with a write cache that works to make writing suck less.
I had one SD card that had a 256 MB cache, though when that was full,
data crawled along at 1-2MB/s.  It also seemed to take forever to
actually commit the cache to disk, telling the OS it had finished
copying the files long before the card could be unmounted.  When copying
the FreedomMaker image to my card, I corrupted a file system or two
before I realized that, no, umount wasn't wrong, it was just taking a
while to finish.

I don't know whether microSD cards can even have such a write cache.

> no swap, more tmpfs, perhaps a different file system altogether?
>
> (good notes on flash file systems at
> http://superuser.com/questions/248078/choice-of-filesystem-for-gnu-linux-on-an-sd-card)

That seems worth investigating, but no, I know no tricks.

> And what about apps? Do you use the memory engine with mysql?
> Something clever with /var?

I haven't done much investigation into the area, but I'd also suggest a
ramdisk for data that can be tossed between rewrites, or a RAM-based
faux write-cache.  I saw something interesting on that, somewhere on the
intertubes, a while ago.

Nick
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