Comment 5 for bug 624229

I get this too when I install something while copying files over the network with rsync, to a raid which was on different drives than the root partition. dpkg freezes at the unpacking step for many minutes, even though the file copying is happening on different drives. dpkg should not be waiting on or syncing filesystems other than the one it's writing to.

Maybe bug 570805 should be reopened, or at least the fix for 570805 sounds like the likely cause of this problem. The fix was described as replacing per-file fsync() calls with per-file sync() calls, which would make it wait for all mounted filesystems on the system to sync, rather than only the file that was just written to. If crash-consistency is the concern here, it really should be syncing only once after the entire unpack is complete IMO, to avoid mixing new files with old in the event of power loss, and to narrow the time window in which a power loss could leave their system in such an intermediate state.