ZFS cautions (pool flag additions committed)

[Karl Denninger]

When ZFS code imports are brought forward that add feature flags there 
is a potential time bomb for existing users in that creation of a new 
pool becomes non-revertible (except read-only!) with regard to mounting 
on older revisions of the code.

The same thing happens if you do a "zpool upgrade" of course, but at 
least that's an explicit act.  You might not realize that you're at risk 
on a pool create, however, unless you **carefully** scrutinize the flags 
that were on the last version compared against the current one.

The primary "gotcha" here occurs if you don't upgrade your emergency 
boot media and for some reason you need to boot from a CD or USB key -- 
you can be left SEVERELY screwed, and since -RELEASE is typically not 
rebuilt when this happens if you don't have a second machine laying 
around on which you can build a RELEASE image....

I've caught this twice now since 10.0-RELEASE shipped and, while I 
haven't been bit by it, it serves as a caution because eventually 
someone tracking -STABLE is going to get badly hurt and be left with an 
unrepairable system.  IMHO there should be some sort of notice on the 
list when new zpool feature flags show up so you're fairly warned that 
building a new emergency boot media copy is required if you intend to 
track -STABLE on a continuing basis.

-- 
-- Karl
karl at denninger.net


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