[Dovecot] SSD drives are really fast running Dovecot

[David Woodhouse]

On Sat, 2011-01-15 at 10:41 +0000, Ed W wrote:
> 
> > One of the systems to fail was a firewall running off SSD.
> 
> SSD or CF?

That doesn't make a lot of difference. They're all broadly similar.
There are better devices and worse devices, but they're mostly crap.

And as I said earlier, even if you think you've worked out which is
which, it may change from batch to batch of what is allegedly the *same*
product.

> It would appear it's also possible to damage some flash memory by 
> powering off at the wrong moment? 

Almost all of them will fail hard it you do any serious power-fail
testing on them. It's not a hardware failure; it's just that their
*internal* file system is corrupt and needs a fsck (or just wiping and
starting again). But since you can't *access* the underlying medium, all
you can do is cry and buy a new one.

The fun thing is that their internal garbage collection could be
triggered by a *read* from the host computer, or could even happen
purely due to a timeout of some kind. So there is *never* a time when
you know it's "safe to power off because I haven't written to it for 5
minutes".

Yes, it's perfectly possible to design journalling file systems that
*are* resilient to power failure. But the "file systems" inside these
devices are generally written by the same crack-smoking hobos that write
PC BIOSes; you don't expect quality software here.

By putting a logic analyser on some of these devices to watch what
they're *actually* doing on the flash when they garbage-collect, we've
found some really nasty algorithms. When garbage-collecting, one of them
would read from the 'victim' eraseblock into RAM, then erase the victim
block while the data were still only held in RAM — so that a power
failure at that moment would lose it. And then, just to make sure its
race window was nice and wide, it would then pick a *second* victim
block and copy data from there into the freshly-erased block, before
erasing that second block and *finally* writing the data from RAM back
to it. It's just scary :)

-- 
dwmw2