Home > Articles > Networking > Storage

  • Print
  • + Share This
This chapter is from the book

I/O Flow Through PSA and NMP

In order to understand how I/O sent to storage devices flows through the ESXi storage stack, you first need to understand some of the terminology relevant to this chapter.

Classification of Arrays Based on How They Handle I/O

Arrays can be one of the following types:

  • Active/Active—This type of array would have more than one Storage Processor (SP) (also known as Storage Controller) that can process I/O concurrently on all SPs (and SP ports) with similar performance metrics. This type of array has no concept of logical unit number (LUN) ownership because I/O can be done on any LUN via any SP port from initiators given access to such LUNs.
  • Active/Passive—This type of array would have two SPs. LUNs are distributed across both SPs in a fashion referred to as LUN ownership in which one of the SPs owns some of the LUNs and the other SP owns the remaining LUNs. The array accepts I/O to given LUN via ports on that SP that “owns” it. I/O sent to the non-owner SPs (also known as Passive SP) is rejected with a SCSI check condition and a sense code that translates to ILLEGAL REQUEST. Think of this like the No Entry sign you see at the entrance of a one-way street in the direction opposite to the traffic. For more details on sense codes, see Chapter 7 ’s “LUN Discovery and Path Enumeration” section.
  • Asymmetric Active/Active or AAA (AKA Pseudo Active/Active)—LUNs on this type of arrays are owned by either SP similarly to the Active/Passive Arrays concept of LUN ownership. However, the array would allow concurrent I/O on a given LUN via ports on both SPs but with different I/O performance metrics as I/O is sent via proxy from the non-owner SP to the owner SP. In this case, the SP providing the lower performance metric accepts I/O to that LUN without returning a check condition. You may think of this as a hybrid between Active/Passive and Active/Active types. This can result in poor I/O performance of all paths to the owner SP that are dead, either due to poor design or LUN owner SP hardware failure.
  • Asymmetrical Logical Unit Access (ALUA)—This type of array is an enhanced version of the Asymmetric Active/Active arrays and also the newer generation of some of the Active/Passive arrays. This technology allows initiators to identify the ports on the owner SP as one group and the ports on the non-owner SP as a different group. This is referred to as Target Port Group Support (TPGS). The port group on the owner SP is identified as Active Optimized port group with the other group identified as Active Non-Optimized port group. NMP would send the I/O to a given LUN via a port in the ALUA optimized port group only as long as they are available. If all ports in that group are identified as dead, I/O is then sent to a port on the ALUA non-optimized port group. When sustained I/O is sent to the ALUA non-optimized port group, the array can transfer the LUN ownership to the non-owner SP and then transition the ports on that SP to ALUA optimized state. For more details on ALUA see Chapter 6.

Paths and Path States

From a storage perspective, the possible routes to a given LUN through which the I/O may travel is referred to as paths. A path consists of multiple points that start from the initiator port and end at the LUN.

A path can be in one of the states listed in Table 5.2.

Table 5.2. Path States

Path State

Description

Active

A path via an Active SP. I/O can be sent to any path in this state.

Standby

A path via a Passive or Standby SP. I/O is not sent via such a path.

Disabled

A path that is disabled usually by the vSphere Administrator.

Dead

A path that lost connectivity to the storage network. This can be due to an HBA (Host Bus Adapter), Fabric or Ethernet switch, or SP port connectivity loss. It can also be due to HBA or SP hardware failure.

Unknown

The state could not be determined by the relevant SATP.

Preferred Path Setting

A preferred path is a setting that NMP honors for devices claimed by VMW_PSP_FIXED PSP only. All I/O to a given device is sent over the path configured as the Preferred Path for that device. When the preferred path is unavailable, I/O is sent via one of the surviving paths. When the preferred path becomes available, I/O fails back to that path. By default, the first path discovered and claimed by the PSP is set as the preferred path. To change the preferred path setting, refer to the “Modifying PSA Plug-in Configurations Using the UI” section later in this chapter.

Figure 5.9 shows an example of a path to LUN 1 from host A (interrupted line) and Host B (interrupted line with dots and dashes). This path goes through HBA0 to target 1 on SPA.

Figure 5.9

Figure 5.9. Paths to LUN1 from two hosts

Such a path is represented by the following Runtime Name naming convention. (Runtime Name is formerly known as Canonical Name.) It is in the format of HBAx:Cn:Ty:Lz—for example, vmhba0:C0:T0:L1—which reads as follows:

vmhba0, Channel 0, Target 0, LUN1

It represents the path to LUN 0 broken down as the following:

  • HBA0—First HBA in this host. The vmhba number may vary based on the number of storage adapters installed in the host. For example, if the host has two RAID controllers installed which assume vmhba0 and vmhba1 names, the first FC HBA would be named vmhba2.
  • Channel 0—Channel number is mostly zero for Fiber Channel (FC)- and Internet Small Computer System Interface (iSCSI)-attached devices to target 0, which is the first target. If the HBA were a SCSI adapter with two channels (for example, internal connections and an external port for direct attached devices), the channel numbers would be 0 and 1.
  • Target 0—The target definition was covered in Chapters 3, “FCoE Storage Connectivity,” and 4, “iSCSI Storage Connectivity.” The target number is based on the order in which the SP ports are discovered by PSA. In this case, SPA-Port1 was discovered before SPA-Port2 and the other ports on SPB. So, that port was given “target 0” as the part of the runtime name.

Flow of I/O Through NMP

Figure 5.10 shows the flow of I/O through NMP.

Figure 5.10

Figure 5.10. I/O flow through NMP

The numbers in the figure represent the following steps:

  1. NMP calls the PSP assigned to the given logical device.
  2. The PSP selects an appropriate physical path on which to send the I/O. If the PSP is VMW_PSP_RR, it load balances the I/O over paths whose states are Active or, for ALUA devices, paths via a target port group whose AAS is Active/Optimized.
  3. If the array returns I/O error, NMP calls the relevant SATP.
  4. The SATP interprets the error codes, activates inactive paths, and then fails over to the new active path.
  5. PSP selects new active path to which it sends the I/O.
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020