I have a question for HA scenarios, where multiple hosts connect to a shared 
block device:

AFAIK, Linux buffers blocks from block devices. So if host A reads a block from 
shared storage (e.g. the MBR), and later host B modifies that block, will host 
A still have the last block read in cache? If so there's a big problem.

Consider that you want to mount a non-clustered filesystem alternatingly on two 
hosts (like for a HA failover). If host A used the filesystem, the host B took 
over the filesystem without booting host A, and later host A takes back the 
filesystem from host B, host A might have some obsolete blocks in its buffer.

Does the application have to flush the buffers explicitly, and if so: Where on 
a multipath device (i.e. on every path or only on the multipath device)?

Sorry, if that's not directly connected to iSCSI, but to shared block storage 
in general.


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