“Operating System Tech Support” <firstname.lastname@example.org> wrote in message
- How does it perform in terms of recovering from the system crash?
does it compare to UNIX filesystem? How about journalled filesystem?
It depends by what you mean as “recovering from a system crash”. Many of
the journalling filesystems, use journaling to restore/fix their
Meta data and FS integrity but it does not do data recovery.
I don’t think that’s quite true. Unixes will lose files being written at the
time of crash, just like QNX will.
The problem is that many of the *NIX like FS’s is that they have very
agressive cache mechanisms to avoid disk access. This is great for
use, but if the system goes down, before those caches are flushed and
commited, you data is gone.
Hmm. That’s not really as much ‘aggressive caching’ as well designed buffer
cache system. In most modern Unixes buffer cache and VM are unified and OS
can automatically adjust buffer cache as usage pattern suggests. Granted,
that (unification) may have adverse effect on realtime, but in QNX they are
not only non-unified, but every devb-xxx has its own cache. That does not
help performance at all and does not help reliability as far as I can see.
In short, the QNX FS might not be as fast as some of the journaled FS/*NIX
but it’s very reliable.
It is reliable, but not really so much more reliable than modern UFS
variants to justify such poor piss performance anymore. I spent lot of
efforts trying to get maximum performance of it and results were at best
Truth is, QNX4 FS was good enough in its days, but by now is simply too damn
old. It does not reflect any of developments in this area of past decade
(which in computer industry means it is relict). Write performance is
especially bad and it also seems to suffer severely from cache saturation
effects. Under some conditions throughput can drop as low as 200kb/sec when
copying a large file within same drive, and that is using DMA mode of EIDE
It is really sad part of QNX (along with VM) and I suggest you guys stop
using excuses already and do something about it. Claims of ‘leading realtime
OS’ and ‘most advanced architecture’ are starting to look somewhat
P.S. cross-posted to OS group where it belongs