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<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">I ran into a strange (to me) issue while testing something in LVM – I wanted to check that zeroing a Logical Volume would really zero the mapped Physical Extents. My setup was simple
– I had a single Volume Group (vg0) containing a single Physical Volume (/dev/mmcblk0p2) and a few Logical Volumes. Nothing fancy.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">The test itself was also simple:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l5 level1 lfo1"><![if !supportLists]><span style="font-family:"Georgia","serif";color:#111111"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Write a test pattern to an LV using ‘dd’<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l5 level1 lfo1"><![if !supportLists]><span style="font-family:"Georgia","serif";color:#111111"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Check how the LV is mapped with ‘pvdisplay -m’<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l5 level1 lfo1"><![if !supportLists]><span style="font-family:"Georgia","serif";color:#111111"><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Read back the mapped PEs from the PVs block device with ‘dd’, checking that they match the test pattern written to the LV<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">It seems like Linux’s disk cache doesn’t like this test. If I follow the steps, the readback from the PV’s block device returns stale data. But if I empty my cache (‘echo 3 > /proc/sys/vm/drop_caches’)
and do the readback again, I can see the pattern that was written to the LV. <o:p>
</o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">Is this the expected behavior? I don’t know much about Linux subsystems, but I’d assume that when LVM maps LV -> PV and does a write, Linux would get the memo and update its disk
cache accordingly. The results I’m seeing suggest that Linux doesn’t update the cache or flag it as dirty, because I can repeatedly read back stale data prior to emptying my cache.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">Here are a few details about my setup:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Platform: Zynq 7000 (Dual-Core ARM Cortex-A9)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Linux: 4.0.0 (built from Xilinx repo)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">LVM Version: 2.02.162(2) (2016-07-28)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">LVM Library Version: 1.02.132 (2016-07-28)<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">LVM Driver Version: 4.30.0<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo6"><![if !supportLists]><span style="font-family:Symbol;color:#111111"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Georgia","serif";color:#111111">Test Media: eMMC device<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111">I don’t know if I’m misunderstanding how LVM/Linux are supposed to mingle or if this is an actual issue.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Georgia","serif";color:#111111"><o:p></o:p></span></p>
<p class="MsoNormal"><b><span style="font-family:"Georgia","serif";color:#111111">Matthew Kipper</span></b><o:p></o:p></p>
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