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Dell SRDF Introduction

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Write operations in asynchronous mode

When SRDF/A is in use, the primary array collects host write operations into delta sets and transfers them in cycles to the secondary array. The primary array acknowledges the host write operations as soon as they are written to its cache.

SRDF/A sessions behave differently depending on:

  • Whether they are managed individually (Single Session Consistency (SSC)) or as a consistency group (Multi Session Consistency (MSC)).
    • With SSC, the SRDF group is managed individually. The primary array's operating environment controls cycle switching. SRDF/A cycles are switched independently of any other SRDF groups on any array in the solution. Cycle switching in asynchronous mode has more details.
    • With MSC, the SRDF group is part of a consistency group spanning all associated SRDF/A sessions. SRDF host software coordinates cycle switching to provide dependent-write consistency across multiple sessions, which may also span arrays. The host software switches SRDF/A cycles for all SRDF groups in the consistency group simultaneously. SRDF/A MSC cycle switching has more details.
  • The number of transmit cycles supported at the R1 side. Enginuity 5876 supports only a single cycle. PowerMaxOS and HYPERMAX OS support multiple cycles queued to be transferred.

Data in a delta set is processed using four cycle types:

  • Capture cycle—Incoming I/O is buffered in the capture cycle on the R1 side. The host receives immediate acknowledgment.
  • Transmit cycle—Data collected during the capture cycle is moved to the transmit cycle on the R1 side.
  • Receive cycle—Data is received on the R2 side.
  • Apply cycle—Changed blocks in the delta set are marked as invalid tracks and destaging to disk begins.

A new receive cycle is started.

The operating environment running on the R1 side determines when the next capture cycle can begin. It also determines the number of cycles that can be in progress simultaneously.

PowerMaxOS 10 (6079), PowerMaxOS 5978, or HYPERMAX OS 5977 – Multicycle mode

There are two situations when SRDF/A operates in multicycle mode:

  • Any array in the configuration runs PowerMaxOS 10 (6079)
  • Both arrays in the configuration run PowerMaxOS or HYPERMAX OS

In multicycle mode, there can be two or more cycles on the R1 side, but only two cycles on the R2 side:

  • On the R1 side:
    • One Capture
    • One or more Transmit
  • On the R2 side:
    • One Receive
    • One Apply

Cycle switches are decoupled from committing delta sets to the next cycle.

When the preset Minimum Cycle Time is reached, the R1 data collected during the capture cycle is added to the transmit queue. Then a new R1 capture cycle begins. There is no wait for the commit on the R2 side before starting a new capture cycle.

The transmit queue holds cycles waiting to be transmitted to the R2 side. Data in the transmit queue is committed to the R2 receive cycle when the current transmit cycle and apply cycle are empty.

Queuing enables smaller cycles of data to be buffered on the R1 side and reduces the size of delta sets transferred to the R2 side.

The SRDF/A session can adjust to accommodate changes in the solution. If the SRDF link speed decreases or the apply rate on the R2 side increases, more SRDF/A cycles can be queued on the R1 side.

Multicycle mode increases the robustness of the SRDF/A session and reduces spillover into the DSE storage pool.

Enginuity 5876

If either array in the solution is running Enginuity 5876, SRDF/A operates in legacy mode. There are two cycles on the R1 side, and two cycles on the R2 side:

  • On the R1 side:
    • One Capture
    • One Transmit
  • On the R2 side:
    • One Receive
    • One Apply

Each cycle switch moves the delta set to the next cycle in the process.

A new capture cycle cannot start until both the transmit cycle on the R1 side and the apply cycle on the R2 side are complete.

Cycle switching can occur within the preset Minimum Cycle Time. However, it can also take longer since it depends on both:

  • The time taken to transfer the data from the R1 transmit cycle to the R2 receive cycle
  • The time taken to destage the R2 apply cycle

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