At 11:48 AM 8/2/01 -0400, Jay R. Ashworth wrote:
>On Wed, Aug 01, 2001 at 06:53:10PM -0700, Jeffrey Siegal wrote:
[snip]
>Unless the machine dies in the middle of a write to one of the files.
>
>Oh yeah, the filesystem will be consistent, but you know what? I don't
>know many suits who give a crap. They want data *not to be lost*, and
>it is my perecption that they're being led to believe that journalling
>filesystems do this.
Define "not to be lost". If you define "not to be lost" in terms of
"once it is completely written to the disk, it will not be lost", then
a journelling file system will meet the requirement and ext2, VFAT,
etc. will _not_ meet the requirement. Even mounting ext2 or VFAT with
synchronous writes will most likely leave (small) windows of
opportunity where an inopportune power loss will corrupt data on the
disk. You may not see this when testing in the lab, but you _will_ see
it in the field. Trust me on this one :-).
Your "suits" appear to be defining "not to be lost" in terms of once
the data is acquired, it will be written to the disk, guaranteed. This
_requires_, that you have some sort of advanced warning that warns you
in time to write your collected data to the disk, guaranteed, before
power is actually lost. During this time, you cannot collect new data
or, by your "suits" definition, you will have to write that data to the
disk too before power fails. Cool, the "suits" just defined a
perpetual motion machine. We're going to be rich as soon as we get
this implemented!
Power failure warning means the hardware needs to have prescience to
accurately predict that power will be lost in the near future. This is
a lot harder to do well than it appears: in order to not miss a true
power failure, you can end up with an unacceptably large number of
false power loss warnings. Think glitches that are long enough to
trigger your power loss detector but power comes back before you
exhaust your power supply capacitors. Remember how we didn't store
data during an impending power loss? Oops, now we have gaps in our
data collection and the "suits" aren't going to like that either.
gvb
P.S. Prescience is a tough thing to have in machine (reference: the
elevator computers in "The Hitchhiker's Guide to the Galaxy" series).
The following was inserted by a lawyer with too little to do.
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