`<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">On May 7, 2014, at 1:44 AM, Filip Marvan <<a href="mailto:filip.marvan@aira.cz">filip.marvan@aira.cz</a>> wrote:<br><div><br class="Apple-interchange-newline"><blockquote type="cite"><div lang="CS" link="blue" vlink="blue" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="Section1" style="page: Section1;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">Hi Richard,<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">thank you for your reply.<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">1. Workload is still the same or very similar. Zvols, which we deleted from our pool were disconnected from KVM server a few days before, so the only change was, that we deleted that zvols with all snapshots.<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">2. As you wrote, our customers are fine for now :) We have monitoring of all our virtual servers running from that storage server, and there is no noticeable change in workload or latencies.</span></font></div></div></div></blockquote><div><br></div><div>good, then there might not be an actual problem, just a puzzle :-)</div><br><blockquote type="cite"><div lang="CS" link="blue" vlink="blue" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="Section1" style="page: Section1;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"><o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">3. That could be the reason, of course. But in the graph are only data from arcstat.pl script. We can see, that arcstat is reporting heavy read accesses every 5 seconds (propably some update of ARC after ZFS writes data to disks from ZIL? All of them are marked as "cache hits" by arcstat script) and with only few ARC accesses between that 5 seconds periody. Before we deleted that zvols (about 0.7 TB data from 10 TB pool, which have 5 TB of free space) there were about 40k accesses every 5 seconds, now there are no more than 2k accesses every 5 seconds.</span></font></div></div></div></blockquote><div><br></div><div>This is expected behaviour for older ZFS releases that used a txg_timeout of 5 seconds. You should</div><div>see a burst of write activity around that timeout and it can include reads for zvols. Unfortunately, the</div><div>zvol code is not very efficient and you will see a lot more reads than you expect.</div><div> -- richard</div><div><br></div><br><blockquote type="cite"><div lang="CS" link="blue" vlink="blue" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div class="Section1" style="page: Section1;"><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"><o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">Most of our zvols have 8K volblocksize (including deleted zvols), only few have 64K. Unfortunately I have no data about size of the read before that change. But we have two more storage servers, with similary high ARC read accesses every 5 seconds as on the first pool before deletion. Maybe I should try to delete some data on that pools and see what happen with more detailed monitoring.<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">Thank you,<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;">Filip<o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> <o:p></o:p></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="2" color="navy" face="Arial"><span style="font-size: 10pt; font-family: Arial; color: navy;"> </span></font></div><div><div class="MsoNormal" align="center" style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman'; text-align: center;"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"><hr size="2" width="100%" align="center" tabindex="-1"></span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><b><font size="2" face="Tahoma"><span style="font-size: 10pt; font-family: Tahoma; font-weight: bold;">From:</span></font></b><font size="2" face="Tahoma"><span style="font-size: 10pt; font-family: Tahoma;"><span class="Apple-converted-space"> </span>Richard Elling [<a href="mailto:richard.elling@richardelling.com" style="color: blue; text-decoration: underline;">mailto:richard.elling@richardelling.com</a>]<span class="Apple-converted-space"> </span><br><b><span style="font-weight: bold;">Sent:</span></b><span class="Apple-converted-space"> </span>Wednesday, May 07, 2014 3:56 AM<br><b><span style="font-weight: bold;">To:</span></b><span class="Apple-converted-space"> </span>Filip Marvan<br><b><span style="font-weight: bold;">Cc:</span></b><span class="Apple-converted-space"> </span><a href="mailto:omnios-discuss@lists.omniti.com" style="color: blue; text-decoration: underline;">omnios-discuss@lists.omniti.com</a><br><b><span style="font-weight: bold;">Subject:</span></b><span class="Apple-converted-space"> </span>Re: [OmniOS-discuss] Strange ARC reads numbers</span></font><o:p></o:p></div></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> </span></font></div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">Hi Filip,<o:p></o:p></span></font></div><div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> </span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">There are two primary reasons for reduction in the number of ARC reads.<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><span class="apple-tab-span"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> <span class="Apple-converted-space"> </span></span></font></span>1. the workload isn't reading as much as it used to<o:p></o:p></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><span class="apple-tab-span"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> <span class="Apple-converted-space"> </span></span></font></span>2. the latency of reads has increased<o:p></o:p></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><span class="apple-tab-span"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> <span class="Apple-converted-space"> </span></span></font></span>3. your measurement is b0rken<o:p></o:p></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">there are three reasons...<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> </span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">The data you shared clearly shows reduction in reads, but doesn't contain the answers<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">to the cause. Usually, if #2 is the case, then the phone will be ringing with angry customers<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">on the other end.<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> </span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">If the above 3 are not the case, then perhaps it is something more subtle. The arcstat reads<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">does not record the size of the read. To get the read size for zvols is a little tricky, you can<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">infer it from the pool statistics in iostat. The subtleness here is that if the volblocksize is <o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">different between the old and new zvols, then the number of (block) reads will be different<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;">for the same workload.<o:p></o:p></span></font></div></div><div><div style="margin: 0cm 0cm 0.0001pt; font-size: 12pt; font-family: 'Times New Roman';"><font size="3" face="Times New Roman"><span style="font-size: 12pt;"> -- richard</span></font></div></div></div></div></div></blockquote></div><br><div apple-content-edited="true">
<div style="color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px;"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; border-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">--</div><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br></div><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><a href="mailto:Richard.Elling@RichardElling.com">Richard.Elling@RichardElling.com</a><br>+1-760-896-4422<br><br><br></div></span></div></span></div></span></div></span></div>
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