bcache, udev rules and calling blkid

[Rolf Fokkens]

Hi,

On bugzilla there was a brief discussion on how to reduce the number of 
blkid calls during udev rules processing:

https://bugzilla.redhat.com/show_bug.cgi?id=1004693

The general idea was to rely on earlier calls to blkid instead of having 
later rules calling blkid themselves. Specifically for bcache this meant 
relying on 13-dm-disk.rules and 60-persistent-storage-rules, which all 
worked fine until...

...until I tested stacking bcache on top of raid (md). In that situation 
61-bcache.rules was out of luck, because there was no prior call to 
blkid so bcache was not detected. The straight forward solution appears 
to be te call blkid from 61-bcache.rules, this works any way. An 
alternative solution might be to move 61-bcache.rules to 65-bcache.rules 
so it is run after 64-md-raid.rules, that might work.

In general I'm a little uncomfortable with this: relying that other 
rules to call blkid may work most of the time, but not always. If rules 
should rely on prior calls to blkid, wouldn't it be better to call blkid 
as rule 00-blk.rules or so? In that case no other rule would ever have 
to call blkid ever. And yes, it may happen that blkid is sometimes 
called without the output actually being used.

Any advice is appreciated,

Thanks,

Rolf