Thin Metadevices on a VMAX

Question

Fom the documentation, we have :

Thin devices are subject to the same maximum size limit as all other Symmetrix device types. In the DMX platform the limit is 60GB and in the VMAX platform the limit is raised to 240GB. If a larger device is required, striped or concatenated thin metadevices can be formed from multiple thin devices.

Now, imagine that I need a 700 GB virtual device fromn a VMAX :

What is the best choice for the striped thin metadevices to be build ?

3 members x 233,33 GB thin devices

or

4 members x 175 GB thin devices

.... and why ?

Thanks

Robert

seancummins · Accepted Answer

Robert,

Given the choice between 3x233.33GB striped metas and 4x175GB striped metas, I would go with 4x175GB. Powers of two are generally best (e.g. 2,4,8,16) because they tend to be closer matches to host IO sizes, which are also typically powers of two (e.g. 4k, 8k, 16k, 32k, 64k). By having a correlation between meta member counts and host IO sizes, your workload will tend to achieve better balance across front-end device queues.

Thanks,

- Sean

You_Want_How_Ma · Answer

I don't think there is a hard and fast rule as to which will be better because it depends on your workload. Based on what we were told when we first designed our system; the 4 member meta should give you better performance than the 3 member meta. The more meta members you have the more you balancing the load across back-end ports and each device will have its own buffer queues. Of course there is a trade-off, you don't want to create 700 1GB devices. The recommendation we are following is to create 125 GB members for most volumes using the auto meta feature (minimum meta capacity = 262668 cyl, member capacity = 136534 cyl).

Using this standard attempts to balance performance and administration overhead. If you have a very small volume that you know will see heavy traffic you might want to customize that volume rather than use the standard size; for example member size of 25 GB for a 200 GB volume giving you an 8 way meta. The same could apply if you have a very large volume; use 240 GB members to build an 8 TB volume.

I hope this helps.

VMAX

Thin Metadevices on a VMAX

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