Avi Kivity wrote:
Chris Mason wrote:
One problem with the spare
capacity model is the general trend where drives from the same batch
that get hammered on in the same way tend to die at the same time. Some
shops will sleep better knowing there's a hot spare and that's fine by
me.
How does hot sparing help? All your disks die except the spare.
Of course, I've no objection to disk sparing as an additional option;
I just feel that capacity sparing is superior.
For any given set of disks, you "just" need to do the math to compute
the utilized capacity, the expected rate of drive failure, the rebuild
time and then see whether you can recover from your first failure before
a 2nd disk dies.
In practice, this is not an academic question since drives do
occasionally fail in batches (and drives from the same batch get stuffed
into the same system).
I suspect that what will be used in mission critical deployments will be
more conservative than what is used in less critical path systems, but
this will be up to the end user to configure...
ric
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