F20 System Wide Change: SSD cache

[Conrad Meyer]

On Mon, 15 Jul 2013 15:36:06 -0400
DJ Delorie <dj at redhat.com> wrote:

> > it's not going to be shoved down your throat.
> 
> I've found this to be untrue in Fedora.

I think this is disingenuous. Especially at the file-system /
block layer, one can point to numerous examples of new
features *not* forced on users. No one forces you to use
LUKS. No one forces you to use LVM. No one forces you to use
btrfs, ext4, or even ext3. You can still use ext2 if you
like. Tmp-on-tmpfs is a default, and while it's opt-out, you
*can* opt out.

Bcache is not something that will be default, and certainly
not for upgrades from existing systems.

> > > At the very least, this feature should be disabled if
> > > the SSD is the boot/root drive.  When SSDs fail, they
> > > fail completely, and it's irresponsible to cause early
> > > failure on a drive that's critical for booting and OS
> > > operation.
> > 
> > By default, bcache runs a write-through cache -- it only
> > caches clean data. If the caching SSD dies, the bcache
> > layer can just forward requests to spinning drive. No
> > data is lost.
> 
> No, I wasn't worried about the spinny disks.  I was worried
> about the SSD itself, in the case where the SSD hosts both
> boot/root *and* a cache for, say, a /home array.

Ah, I tend to think of /home as living on root.

Yes we agree here. One shouldn't use an SSD for both data
(/boot, /, or otherwise) and cache.

> > > Also, I think such features should be postponed
> > > until/unless there's a clear and obvious way to
> > > configure/disable them that doesn't involve installing
> > > additional packages or editing obscure text files.
> > 
> > Again -- no one is forcing you to use this. It's opt-in.
> 
> Please read the /tmp-on-tmpfs thread for an example of what
> I'm worried about.

*Sigh*. This is pure hyperbole. Even the author of bcache
can't show that bcache gives performance wins on all
systems / use-cases. It's a wash.

And as you're afraid of, there are risks to using it that
shouldn't be the default for users. I agree.

I think bcache should be opt-in but available. I agree that
it should not be the default. It would be nice if anaconda
supported it as an option, but that takes a significant
amount of work.

I'm not convinced this should be a Fedora feature (unless
Anaconda support is there). Until then, it's just a new 3.9
kernel feature.

Conrad