Release Notes

System configuration
7 Dell EMC SC Series: Performance and Availability | 4024-BP-SC
1.2 Maintain the latest release of SCOS
Dell EMC regularly updates Storage Center OS (SCOS) to improve performance, reliability, functionality, and
provide new features. Dell EMC recommends running the latest generally available version on your system.
1.3 SAS cabling and drive placement
SC Series systems are installed with resilient cabling so that each controller has two paths to every disk. In
general, to achieve the best performance from a system, drives are spread across all SAS chains in the
system. Depending on the performance needs for a particular design, you may require one or multiple SAS
chains in the system. It is always best to spread disks across all SAS chains to maximize bandwidth.
1.4 Distributed spare (new in SCOS 7.3)
Beginning with SCOS 7.3, spare disk space is distributed across all drives in a drive folder and is designated
as spare space. Data from a failed drive is moved to available spare disk space. The distributed spare feature
replaces traditional hot-sparing technology used in releases prior to SCOS 7.3. All new systems deployed or
upgraded with SCOS 7.3 use the distributed spare model.
Prior to SCOS 7.3, SC Series arrays required dedicated hot spares, such as full SSDs or HDDs reserved for
receiving rebuild data from another drive that has failed. These spares stood idle until a drive failed in the
system, and they also created a bottleneck during the restore process, since the entire failed drive had to be
restored onto a single spare.
The distributed spare feature provides the following benefits over hot sparing:
• Improved I/O efficiency: All drives now participate in normal user transactions.
• Up to 500% faster rebuilds than hot sparing mode: Failed drive data is now restored in parallel to
multiple drives.
• Reduced risk of data loss: Faster rebuilds reduce time during which data is unprotected.
All systems deployed with SCOS 7.3 are in distributed spare mode and do not have the option to revert to hot
sparing mode. Once the system is upgraded to SCOS 7.3, the UI (Dell EMC Unisphere™ Central, Unisphere,
and Dell Storage Manager) will display a message stating that the distributed spare optimizer is currently
disabled. The optimizer is a background process that moves data and balances disk resources to yield the
best steady-state system performance. When this is turned on for an existing system, data movement
consumes back-end resources and may impact host I/O. For this reason, the optimizer is disabled by default
upon upgrade to SCOS 7.3. The user may choose when to enable or disable the optimizer. Consider the
current I/O activity as well as the amount of drives and capacity that needs to be balanced before enabling the
optimizer. In addition, it is recommended to enable the optimizer during a maintenance window if possible.
1.5 RAID spare ratio of distributed spare
The RAID spare ratio is the number of userdata extents per spare extend in a RAID device. Much like the
previous hot sparing method, the default for distributed sparing is 20 userdata extents per spare extend. Upon
upgrade to SCOS 7.3, the spare ratio is calculated based on the spares and RAID devices that exist at the
time of the upgrade. Existing spare ratios will be honored for upgrades to current systems.