..
On Mon, 30 Mar 2009, Mark Lord wrote:I spent an entire day recently, trying to see if I could significantly fill
up the 32MB cache on a 750GB Hitach SATA drive here.
With deliberate/random write patterns, big and small, near and far,
I could not fill the drive with anything approaching a full second
of latent write-cache flush time.
Not even close. Which is a pity, because I really wanted to do some testing
related to a deep write cache. But it just wouldn't happen.
I tried this again on a 16MB cache of a Seagate drive, no difference.
Bummer. :)
Try it with laptop drives. You might get to a second, or at least hundreds of ms (not counting the spinup delay if it went to sleep, obviously). You probably tested desktop drives (that 750GB Hitachi one is not a low end one, and I assume the Seagate one isn't either).
You'll have a much easier time getting long latencies when seeks take tens of ms, and the platter rotates at some pitiful 3600rpm (ok, I guess those drives are hard to find these days - I guess 4200rpm is the norm even for 1.8" laptop harddrives).
And also - this is probably obvious to you, but it might not be immediately obvious to everybody - make sure that you do have TCQ going, and at full depth. If the drive supports TCQ (and they all do, these days) it is quite possible that the drive firmware basically limits the write caching to one segment per TCQ entry (or at least to something smallish).