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Dell internalises EqualLogic's automated data movement

Better than Compellent, it claims

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Comment Dell contacted us when we reported on its 10GbitE product announcements to say that contrary to our first impression, that it did have "auto-data movement between tiers of EqualLogic storage."

How does this work when EqualLogic arrays have to have homogeneous disk drives inside them?

Compellent has had automated data movement between different performance tiers of storage in its arrays for some time. EMC is adding it, under the FAST moniker, to its arrays. 3PAR is also making moves in that direction, as is Symantec. The general trend to add solid state drives (SSDs) to storage arrays is encouraging vendors to automate the movement of data to and from SSD storage, as its access profile waxes and wanes.

Dell's EqualLogic arrays support different tiers of storage. An EqualLogic array consists of a pair of drive controllers linked to drive enclosures. Such an array is a unit and a unit must use the same disk drives.

A number of arrays can be combined into a group and present a virtualised pool of storage. The arrays in that group can have different tiers of storage, faster or slower drives for example. A customer can set up RAID groups, logical groups of drives sharing a RAID protection scheme, within such a group. The drives within a RAID group must be homogeneous, though.

Different RAID levels have different biases towards performance and protection/availability and capacity. RAID-10 stripes data across the disks in the group on top of RAID 1 (mirrored) sets, providing the highest availability and random write performance but lowest capacity. RAID-5 (distributed parity) has lower availability and random write performance. RAID-50 stripes on top of RAID 5 sets and provides better performance.

Eric Schott, Dell's director of product management, says: "RAID types have a very large impact on performance. A 10K drive running RAID-50 could be slower than a [7200rpm] SATA one running RAID-10."

Separate SAN (storage area network) pools can also be set up across EqualLogic arrays, so that, for example, data from different customer departments is logically separated. Different pools can have different tiers of disk storage - 15K SAS drives or 7,200rpm SATA drives for example - so that apps with different SAN I/O requirements can be allocated to the right pool. Customers can manually move data between pools, but not do it automatically. Schott says such manual moves do not disrupt data access by connected application servers.

EqualLogic arrays carry out load-balancing to optimise performance and Schott says: "We only load balance within a pool," so that I/O burdens into a pool are shared equally across all the array controllers with drives in that pool. After a new volume is first created, it has a performance history accumulated for it. The EqualLogic system may then move the volume within the pool RAID group so as to optimise its performance.

It is not moved from one drive type to another, though. There would be no such automated movement from a SATA drive-based pool to a faster SAS drive-based pool and mixed-drive type pools are not allowed.

Schott said: "We are absolutely not looking at the disk type level, SAS or SATA."

Dell competitor Compellent has automated data movement at the block level, which is not something that Dell's EqualLogic arrays do. But Dell asserts that it does not matter from a performance point of view, with Schott saying: "We don't lose in my experience to Compellent based on performance. We do much better than them on overall performance per drive. Candidly, I haven't run into them beating us or even being close to us [on performance]."

Our understanding is that an EqualLogic development may be coming in 2010 which could involve EqualLogic controllers having several storage tiers below them. Whether it would also involve automated data movement between storage tiers remains to be seen. ®

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