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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>&nbsp;</o:p></p>

<p class=MsoNormal>For future server purchases, I want to know which is
faster:&nbsp; (a) A bunch of hard disks with PERC and WriteBack enabled, or (b)
A bunch of hard disks, plus one SSD for ZIL.&nbsp; Unfortunately, I don&#8217;t
have an SSD available for testing.&nbsp; So here is what I was able to do:<o:p></o:p></p>

<p class=MsoNormal><o:p>&nbsp;</o:p></p>

<p class=MsoNormal>I measured the write speed of the naked disks (PERC set to
WriteThrough).&nbsp; Results were around 350 ops/sec.<o:p></o:p></p>

<p class=MsoNormal>I measured the write speed with the WriteBack enabled.&nbsp;
Results were around 1250 ops/sec.<o:p></o:p></p>

<p class=MsoNormal><o:p>&nbsp;</o:p></p>

<p class=MsoNormal>So right from the start, we can see there&#8217;s a huge
performance boost by enabling the WriteBack.&nbsp; Even for large sequential
writes, buffering allows the disks to operate much more continuously.&nbsp; The
next question is how it compares against the SSD ZIL.<o:p></o:p></p>

<p class=MsoNormal><o:p>&nbsp;</o:p></p>

<p class=MsoNormal>Since I don&#8217;t have an SSD available, I created a ram
device and put the ZIL there.&nbsp; This is not a measure of the speed if I had
an SSD; rather, it is a measure which the SSD cannot possibly achieve.&nbsp; So
it serves to establish an upper bound.&nbsp; If the upper and lower bounds are
near each other, then we have a good estimate of the speed with SSD ZIL &#8230;
but if the upper bound and lower bound are far apart, we haven&#8217;t gotten
much information.<o:p></o:p></p>

<p class=MsoNormal><o:p>&nbsp;</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>&nbsp;</o:p></p>

<p class=MsoNormal>This measure is very far from the lower bound.&nbsp; But it
still serves to provide some useful knowledge.&nbsp; 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'>&middot;<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
</span></span></span><![endif]>There&#8217;s a lot of performance to be gained
by accelerating the ZIL.&nbsp; 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'>&middot;<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
</span></span></span><![endif]>The WriteBack raid controller achieves a lot of
this performance increase.&nbsp; 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'>&middot;<span style='font:7.0pt "Times New Roman"'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
</span></span></span><![endif]>I don&#8217;t know how much an SSD would
help.&nbsp; I don&#8217;t know if it&#8217;s better, the same, or worse than
the PERC.&nbsp; I don&#8217;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>&nbsp;</o:p></p>

<p class=MsoNormal>I have a hypothesis.&nbsp; 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.&nbsp;
This has yet to be proven or disproven.<o:p></o:p></p>

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