Tuesday, July 15, 2014

EMC VMAX Virtual Provisioning: Striped Metas now generally recommended

For some reason, I find it very easy to come across REV A12 of the EMC® Symmetrix®Virtual Provisioning™ documentation referenced below compared to REV A14.  Putting up this blog post more for myself than others... perhaps this way I'll easily be able to track down REV A14.  For me REV A14 is important because its when the general recommendation switched from concatenated metas to striped metas.  No doubt the performance benefits were the motivation, and EMC waited until enough operations on striped metas were possible to ease administrative burden of recommending striped metas.

I find the recommendation of striped metas especially important for Oracle redo logs and SQL Server transaction logs when SRDF synchronous replication is used.  This is because the serialization required for replication has less of a performance drag with striped metas than concatenated metas. Striped metas may also help when asynchronous replication is used, because of the write cache policies on the VMAX.

Best Practices for Fast, Simple Capacity Allocation with EMC® Symmetrix®Virtual Provisioning™

In most cases, EMC recommends using concatenated rather than striped metadevices with Virtual Provisioning.
Page 12

Best Practices for Fast, Simple Capacity Allocation with EMC® Symmetrix®Virtual Provisioning™
Document 7383 300-006-718 REV A14  (link requires EMC Account login)

In most cases, EMC recommends using striped rather than concatenated metadevices with Virtual Provisioning because they offer the following advantages:

  • With Synchronous SRDF®, Enginuity allows one outstanding write per thin device per path. With concatenated metadevices, this could cause a performance problem by limiting the concurrency of writes. 
    This limit will not affect striped metadevices in the same way because of the small size of the metavolume stripe (1 cylinder or 1920 blocks).
  • In Enginuity releases prior to 5875, the code allows eight read requests per path per thin device. This may limit the number of read requests that can be passed through to the thin pool regardless of the number of data devices it contains. This can cause slower performance in environments with a high read miss rate. A striped metadevice offers improved throughput, because each metamember is allowed to queue requests to the backend in parallel. Even with the limit removed in Enginuity 5875, there are still potential performance benefits to using striped thin metas over concatenated thin metas.
  • Symmetrix Enginuity has a logical volume write pending limit to prevent one volume from monopolizing writeable cache. Because each metamember gets a small percentage of cache, a striped meta is likely to offer more writeable cache to the metavolume.
However, concatenated thin metadevices do offer two important operational advantages over striped thin metadevices:

  • Non-metadevices can be converted to concatenated metadevices without destroying the existing data. Beginning with Enginuity 5875 and Solutions Enabler 7.2, it’s possible to convert a thin device into a concatenated meta without unmapping the device first.
  • Concatenated thin metadevices can be expanded without destroying existing data by adding metamembers to an existing metadevice. Beginning with Enginuity 5875, a striped metadevice can also be expanded while preserving existing data. 
    Pages 12-13

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