Yeah, at least on the RAID equipment I use basicly has some sort of
standard RAM (SIMM/DIMM) that it uses for caching. There is an option to
connect a battery to the controller. If the power fails while there is
unwritten data in the cache it can be flushed to disk when the power
returns.
Using EEPROM memory as a logging device wouldn't be a good idea. These
devices tend to have a limited number of rewrites and will begin to
introduce bit errors as they wear out.
Question: On a journaling file system do you want unwritten journal data
written to disk after power is restored?
I guess if the OS thinks it has hit the physical disk then it would be
fine to commit it. So what if you didn't have a battery backed cache, and
data the OS thought was on disk didn't make it?
Should a journaled FS always have uncorrupt data in the files no matter
what state the various caches are in?
(I'm just trying to get my mind wrapped around this stuff, it is kinda new
to me.)
On Fri, 9 Jun 2000, Andreas Dilger wrote:
> Xuan, you write:
> > Is it reasonable to use the 50 bytes of /dev/nvram for logging or is it just
> > too small?
>
> It is definitely too small to write any transactions there, although it
> may be possible to use it for a bitmap of some sort (400 bits). However,
> my understanding would be that the CMOS NVRAM would be much too slow to
> use reasonably, and it only has a limited number of writes, so using it
> for part of a filesystem will surely mean death for it. Correct me if
> I'm wrong for modern motherboard NVRAM.
>
> The NVRAM that is being referred to is usually battery-backed RAM, so
> it is very fast, can handle lots of write cycles, and has a fairly long
> lifetime when the power is disconnected (uses rechargeable batteries).
>
> Cheers, Andreas
>
--
...and sometimes, late at night I get these twitches. Like dead people get.
(Or, as I prefer to call them, perfect computer users)
--The BOFH
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