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Showing posts from July 12, 2015

Storage I/O Control (SIOC)

SIOC VMware vSphere Storage I/O Control (SIOC) provides I/O prioritization of virtual machines running on a cluster of ESXi hosts with access to shared storage. It extends the constructs of shares and limits, which existed for CPU and memory, to manage storage utilization. Use SIOC to configure rules and policies to specify the business priority of each virtual machine using shares and limits. When I/O congestion is detected, Storage I/O Control dynamically allocates the available I/O resources to virtual machines according to the rules and policies, improving service levels and consolidation ratios. At a basic level SIOC is monitoring the end to end latency of a datastore. When there is congestion, SIOC reduces the latency by throttling back virtual machines that are using excessive I/O. SIOC will use the share values assigned to the virtual machine’s VMDKs to prioritize access to the datastore. The purpose of SIOC is to address the noisy neighbor problem, i.e. a l

Universally Unique Identifier (UUID)

UUID A Universally Unique IDentifier (UUID) is a 16-octet (128-bit) number. In its canonical form, a UUID is represented by 32 lowercase hexadecimal digits, displayed in five groups individually separated by hyphens, in the form 8-4-4-4-12. In general, UUID is used to uniquely identify an object or entity on the Internet. VMware storage architecture has multiple, unique identifiers: NAA & EUI (most common): Network Address Authority  & Extended Unique Identifier Guaranteed to be unique to the LUN The preferred method of identifying LUNs Generated by the storage device MPX  (local datastores): For devices that do not provide an NAA number, ESXi generates an MPX identifier Represents the local LUN or disk Takes the form mpx.vmhba<Adapter>:C<Channel>:T<Target>:L<LUN>, e.g. mpx.vmhba33:C0:T1:L0 Can be used in the exact same way as the NAA identifier VML: Can be used interchangeably with the NAA identifier and the MPX identi

SCSI Reservations & Atomic Test and Set (ATS)

SCSI Reservations SCSI reservations are used to control access to a shared SCSI device such as a LUN. An initiator or host sets a reservation/lock on a LUN in order to prevent another host from making changes to it. This is similar to the file-locking concept. A SCSI reservation conflict occurs if a host tries to access a datastore that was locked by another host. A Logical Unit Number (LUN) is an individual, unique, block-based storage device, the term LUN is often used interchangeably with disk and datastore, depending on the context. In a shared storage environment, when multiple hosts access the same Virtual Machine File System (VMFS) datastore, specific locking mechanisms are used. These locking mechanisms prevent multiple hosts from concurrently writing to the metadata and ensure that no data corruption occurs. SCSI Reservations SCSI reservation is a technique that manages disk contention by preventing I/O on an entire LUN for any ESXi host or VMs (other