access to hard drives is "blocked" by writes to a flash drive

[Peter Jeremy]

On 2013-Mar-03 23:12:40 -0800, Don Lewis <truckman at freebsd.org> wrote:
>On  4 Mar, Konstantin Belousov wrote:
>> It could be argued that the current typical value of 16MB for the
>> hirunningbufspace is too low, but experiments with increasing it did
>> not provided any measureable change in the throughput or latency for
>> some loads.
>
>The correct value is probably proportional to the write bandwidth
>available.

The problem is that write bandwidth varies widely depending on the
workload.  For spinning rust, this will vary between maybe 64KBps
(512B random writes) and 100-150MBps (single-theaded large sequential
writes).  The (low-end) SSD in my Netbook also has about 100:1 variance
due to erase blocking.  How do you tune hirunningbufspace in the face
of 2 or 3 orders of magnitude variance in throughput?  Especially since
SSDs don't gradually degrade - they hit a brick wall.

-- 
Peter Jeremy
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