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<p class=MsoNormal>In this email, when I say PERC, I really mean either a PERC,
or any other hardware WriteBack buffered raid controller with BBU.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>For future server purchases, I want to know which is
faster: (a) A bunch of hard disks with PERC and WriteBack enabled, or (b)
A bunch of hard disks, plus one SSD for ZIL. Unfortunately, I don’t
have an SSD available for testing. So here is what I was able to do:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I measured the write speed of the naked disks (PERC set to
WriteThrough). Results were around 350 ops/sec.<o:p></o:p></p>
<p class=MsoNormal>I measured the write speed with the WriteBack enabled.
Results were around 1250 ops/sec.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>So right from the start, we can see there’s a huge
performance boost by enabling the WriteBack. Even for large sequential
writes, buffering allows the disks to operate much more continuously. The
next question is how it compares against the SSD ZIL.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Since I don’t have an SSD available, I created a ram
device and put the ZIL there. This is not a measure of the speed if I had
an SSD; rather, it is a measure which the SSD cannot possibly achieve. So
it serves to establish an upper bound. If the upper and lower bounds are
near each other, then we have a good estimate of the speed with SSD ZIL …
but if the upper bound and lower bound are far apart, we haven’t gotten
much information.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>With the ZIL in RAM, results were around 1700 ops/sec.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This measure is very far from the lower bound. But it
still serves to provide some useful knowledge. The take-home knowledge
is:<o:p></o:p></p>
<p class=MsoListParagraph style='margin-left:.25in;text-indent:-.25in;
mso-list:l0 level1 lfo2'><![if !supportLists]><span style='font-family:Symbol'><span
style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]>There’s a lot of performance to be gained
by accelerating the ZIL. Potentially up to 6x or 7x faster than naked
disks.<o:p></o:p></p>
<p class=MsoListParagraph style='margin-left:.25in;text-indent:-.25in;
mso-list:l0 level1 lfo2'><![if !supportLists]><span style='font-family:Symbol'><span
style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]>The WriteBack raid controller achieves a lot of
this performance increase. About 3x or 4x acceleration.<o:p></o:p></p>
<p class=MsoListParagraph style='margin-left:.25in;text-indent:-.25in;
mso-list:l0 level1 lfo2'><![if !supportLists]><span style='font-family:Symbol'><span
style='mso-list:Ignore'>·<span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]>I don’t know how much an SSD would
help. I don’t know if it’s better, the same, or worse than
the PERC. I don’t know if the combination of PERC and SSD together
would go faster than either one individually.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I have a hypothesis. I think the best configuration
will be to use a PERC, with WriteBack enabled on all the spindle hard drives,
but include an SSD for ZIL, and set the PERC for WriteThrough on the SSD.
This has yet to be proven or disproven.<o:p></o:p></p>
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