The short story - EMC doesn’t dump cache nor write through when the CX4 loses an SP (either via failure or NDU).

It’s a big deal as everything stops when the CX dumps cache and then writes slow down (and possibly reads as a consequence of no write consolidation) without WCA.

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With WCA, there is no cache dump, so write back continues and performance should be about the same with 1 SP as with 2 (unless IO load exceeds capacity of one SP – which is unlikely

EMC expects the bigger CLARiiON shops will see a positive impact with WCA

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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small;"><span style="font-family: Times New Roman;"><span style="text-decoration: underline;">What is WCA?</span><strong style="mso-bidi-font-weight: normal;"></strong></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">Write Cache Availability refers to a change in how CX4 and FLARE release 28 implements cache management. CLARiiON storage systems prior to R28 have automatically destaged write cache data to triple-mirrored vault drives when a variety of components failed. With WCA, data will now be maintained in cache in the following failure scenarios:</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Single SP failure</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Single vault drive failure</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Power supply failure<span style="mso-spacerun: yes;">  </span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">SPS failure</span></span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">Cache destage to the vault drives will continue to operate as it always has in the following cases:</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Loss of power to the array on both the A and B power circuits</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Array overheating conditions affecting both SPs</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Multi-fan failures (on CX4-80)</span></span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">Maintenance procedures that triggered automatic cache destage will no longer do so. </span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Non-disruptive Upgrade (NDU) of FLARE and layered applications</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Single SP restart</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Single SP physically removed</span></span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">What has changed since Cache Destage to Vault was introduced?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">Patented in 1994, the CLARiiON disk array had many features that are now standard in the industry. Features mentioned in early patents included the industry's first dual active controller design, full system redundancy, and mirrored write cache. To protect the integrity of data in cache, this early design introduced cache destage to the vault drives. CLARiiON also included an innovative RAID architecture to protect in flight writes to RAID groups in the presence of power failures. Subsequent innovations improved on this design, e.g. algorithms to optimize the performance of cache destage process.</span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small;"><span style="font-family: Times New Roman;">In the ensuing years, CLARiiON customers benefited from continuous improvements in CLARiiON hardware, and software architecture. In May 2006, EMC introduced the CX3 UltraScale family of storage systems. Detailed analysis of CX3 systems in the field showed that these systems were delivering 99.999% uptime performance. After reviewing and analyzing detailed reports on over 10,000 CX3 systems in the field, CLARiiON engineering determined that data could safely be maintained in write cache in specific failure scenarios.<span style="mso-spacerun: yes;">  </span></span></span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">Why is WCA being implemented?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">The analysis of thousands of CX3 UltraScale systems proved that this could be done now with a very high degree of safety. WCA will provide important benefits to CLARiiON customers:</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Users will benefit from improved application performance immediately following certain component failures.<span style="mso-spacerun: yes;">  </span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">In high-end CLARiiON arrays, customers will be able to utilize more cache for writes. Larger write cache will provided improved performance, particularly for applications that perform many random write operations such as data base applications and Exchange.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Array performance will be improved during hardware maintenance procedures such replacing an SP, power supply, or SPS.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">Array performance will be improved during NDU maintenance procedures</span></span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">What will happen when a power supply, blower, or SPS fails?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">These components are redundant and hot swappable. The storage system will continue to operate normally if any of these components should fail. CLARiiON provides the means for customers to be notified when failure event occurs. Through CLARalert, the EMC Customer Services will also be notified of the event. The best practice has always been to replace the failed component as soon as possible. The probability of two similar components failing within a small time window is extremely low. </span></p>
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<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">What will happen when an SP fails or is removed for maintenance?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">Data will remain in the write cache of the surviving SP thereby increasing the performance of the array. This is called “single-board write caching”. Consider a situation where SP-A is the surviving storage processor. SP-A will detect that the peer has panicked or been removed and transition to single-board write caching without increasing response times for I/O requests that are currently in flight. When SP-B is rebooted, the write cache image in SP-A will be written to SP-B.</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;"> </span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">What will happen when one SP panics while the other is down?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">FLARE is designed to react to a variety of failure scenarios. These will be covered in detail in an engineering white paper. Some examples:</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">When a surviving SP panics the, write cache memory will be persisted through the reboot and as soon as the SP comes up, the cache will be rebuilt and then enabled.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">If both SPs panic almost simultaneously, then the SP that comes up first will send its copy of the write cache image to the second SP.</span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt 0.25in; text-indent: -0.25in; tab-stops: list .25in;"><span style="font-family: Symbol; mso-fareast-font-family: Symbol; mso-bidi-font-family: Symbol;"><span style="font-size: small;">·</span></span><span style="font-family: Times New Roman;"><span style="font-size: 7pt; mso-fareast-font-family: Symbol;">        </span><span style="font-size: small;">While an SP is rebooting, if the surviving SP panics, then the cache will be disabled until the second SP (original surviving SP) comes up. The cache image will be check and then transferred to the peer. </span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small;"><span style="font-family: Times New Roman;"><span style="mso-spacerun: yes;"> </span>. </span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">What do customers need to do when configuring a CX4 array?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">There is no setup involved. WCA is simply part of the built-in functionality for CX4 series storage systems. Cache tuning and cache management functions are not changing.</span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;"> </span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="text-decoration: underline;"><span style="font-size: small;"><span style="font-family: Times New Roman;">Are there special considerations for performing maintenance or upgrades?</span></span></span></p>
<p class="MsoNormal" style="margin: 0in 0in 0pt;"><span style="font-size: small; font-family: Times New Roman;">EMC has added an illuminated hand with red slash through it on all SPs to clearly indicate which SP is active and which isn’t to avoid SP replacement gaffes. In addition, the CLARiiON Procedure Generator will also continue to generate tested and approved maintenance procedures. </span></p>
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