On 06/05/2010 15:31, Tomas Ögren wrote:
On 06 May, 2010 - Bob Friesenhahn sent me these 0,6K bytes:
On Wed, 5 May 2010, Edward Ned Harvey wrote:
In the L2ARC (cache) there is no ability to mirror, because cache device
removal has always been supported. You can't mirror a cache device, because
you don't need it.
How do you know that I don't need it? The ability seems useful to me.
The gain is quite minimal.. If the first device fails (which doesn't
happen too often I hope), then it will be read from the normal pool once
and then stored in ARC/L2ARC again. It just behaves like a cache miss
for that specific block... If this happens often enough to become a
performance problem, then you should throw away that L2ARC device
because it's broken beyond usability.
Well if a L2ARC device fails there might be an unacceptable drop in
delivered performance.
If it were mirrored than a drop usually would be much smaller or there
could be no drop if a mirror had an option to read only from one side.
Being able to mirror L2ARC might especially be useful once a persistent
L2ARC is implemented as after a node restart or a resource failover in a
cluster L2ARC will be kept warm. Then the only thing which might affect
L2 performance considerably would be a L2ARC device failure...
--
Robert Milkowski
http://milek.blogspot.com
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