On 10-Feb-09, at 1:05 PM, Peter Schuller wrote:
YES! I recently discovered that VirtualBox apparently defaults to
ignoring flushes, which would, if true, introduce a failure mode
generally absent from real hardware (and eventually resulting in
consistency problems quite unexpected to the user who carefully
configured her journaled filesystem or transactional RDBMS!)
I recommend everyone to be extremely hesitant to assume that any
particular storage setup actually honors write barriers and cache
flushes. ...
+1.
You need the underlying device to do the right thing, the driver to do
the right thing, the operating system in general to do the right thing
(which includes the file system, block device layer if any etc - for
example, if use md on Linux with RAID5/6 you're toast).
Absolutely.
So again I cannot stress enough - do not assume things behave in a
non-broken fashion with respect to write barriers and flushes.
That's why I believe there is no substitute for pull-plug tests, and
I would perform quite a few on a loaded system before being confident
about it.
The last time I did that in anger was against a Sun X2200 + LVM
mirror + Ubuntu + reiser3fs + MySQL InnoDB, and it performed
flawlessly (although I agree there may be a weak link in LVM; not my
choice. I'd have chosen Solaris+ZFS).
I can't
speak to expensive integrated hardware solutions; I HOPE, though at
this point my level of paranoid does not allow me to assume, that if
you buy boxed systems from companies like Sun/HP/etc you get decent
stuff. But I can definitely say that paying non-trivial amounts of
money for hardware is not a guarantee that you won't get completely
broken behavior.
+1.
--Toby
...
--
/ Peter Schuller
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