On Tue, 4 Mar 2008, Ivan Voras wrote:
I'm curious about the math behind this - is ~4000 burst or sustained
rate? For common BBU cache sizes (256M, 512M), filling that amount with
data is pretty trivial. When the cache is full, new data can enter the
cache only at a rate at which old data is evacuated from the cache (to
the drive), which is at "normal", uncached disk drive speeds.
Should be sustained rate. The reason is if you have no BBU cache, then
each transaction needs to wait for the disc to rotate around to the bit
where you want to write, even though each transaction is going to be
writing in approximately the same place each time. However, with a BBU
cache, the system no longer needs to wait for the disc to rotate, and the
writes can be made from the cache to the disc in large groups of
sequential writes, which is much faster. Several transactions worth can be
written on each rotation instead of just one.
Matthew
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