[Sugar-devel] Adventures in the land of ENOSPC

[Sascha Silbe]

Martin Langhoff <martin.langhoff at gmail.com> writes:

> Manuel Kaufmann has been looking at SL#394, and looking at the bug
> report, it struck me that it was reported backwards. I would have
> written: "I filled up my disk and it knocked the system out, Sugar
> would not start again, etc. Oh, btw, it was with Browse".

I agree. Every single mishandling of ENOSPC is a bug in the affected
piece of software and should be fixed. However, there's a lot of broken
software (including Sugar) out there that doesn't always do the right
thing, often resulting in data loss (because files are updated
in-place). And there will always be new software that's broken in this
regard. It's the responsibility of system designers to reduce the risk
of this happening by avoiding an ENOSPC situation wherever reasonably
possible.

Browse in particular is a very good candidate for avoiding
ENOSPC. Usually the user is not going to be interested in a partial
download and the file size is known in advance. It doesn't make sense to
even start a download we already know to never complete
successfully.

Similarly, the data store should take care to leave some free
space. Maybe even some heuristic that somehow rejects easy-to-regenerate
entries earlier than others, if we can come up with a good enough one.


[...]
> I artificially filled up / with dd until barely 1MB was remaining,
> then experimented with filling it up completely, then leaving a few K
> free and saving stuff.

Yes, I fully expect it to break left and right, like it always breaks in
situations that are somewhat on the side of the road. We only test the
"good" cases and a few failure scenarios the patch author thinks about
(if any). There are no systematic tests that cover the less common
failure modes. We don't even have a list of scenarios that we care
about. We don't do system design where these kinds of things are
considered.


[...]
>  - Next boot, my Journal was gone. Actually, it's not gone, I can see
> the files in ~/.sugar/default/datastore, but it is corrupt or [...]

Sounds like SL#2317 [1], as Gonzalo already pointed out.

Sascha

[1] https://bugs.sugarlabs.org/ticket/2317
-- 
http://sascha.silbe.org/
http://www.infra-silbe.de/
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 489 bytes
Desc: not available
URL: <http://lists.sugarlabs.org/archive/sugar-devel/attachments/20120915/41da1591/attachment.pgp>