zfetch: Don't issue new streams when old have not completed

The current dmu_zfetch code implicitly assumes that I/Os complete
within min_sec_reap seconds. With async dmu and a readonly workload
(and thus no exponential backoff in operations from the "write
throttle") such as L2ARC rebuild it is possible to saturate the drives
with I/O requests. These are then effectively compounded with prefetch
requests.

This change reference counts streams and prevents them from being
recycled after their min_sec_reap timeout if they still have
outstanding I/Os.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Matt Macy <[email protected]>
Closes #10900 

0 files changed, 0 insertions, 0 deletions