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Direct-Attach Storage

Direct-Attach storage will employ both RAID Controllers (or HBA) and disk enclosures. Now that SAS on the back-end of entry and mid-range SAN systems is becoming popular, the disk enclosures may be the same for both SAN and direct-attach.

Disk Enclosures

The next step in the chain of devices from the system IO bus to the disk drive is the disk enclosure (EMC uses the term DAE, which will also be used here even for non-EMC enclosures). Some years ago, a 3U enclosure for 15 3.5in disk drives was the prevalent standard configuration. Variations included a 14 and 16 3.5in disk enclosures.

HP may have been the first major vendor to switch to a 2U 12-disk enclosure for 3.5in drives. This is form-factor for the HP StorageWorks 60 Modular Smart Array (also MSA60). The new HP name is now StorageWorks D2600.


The first enclosure for 2.5in disks was 1U 10-disk MSA50, but this seems to be nolonger actively sold.


The standard configuration for 2.5in drives seems to be a 2U enclosure for 24 disks as employed in the StorageWorks 2000 series.


The 2U 25 drive form factor was used for the original MSA70, and now the D2700. (Is old MSA 60/70 name for 3Gbps SAS only? and the new D2600/2700 for 6Gbps SAS?)



And the legacy 15 LFF (3.5in) disk enclosure.


My view is that the 12-disk enclosure is better matched to a single IO channel than the 15-disk enclosure. The 24 disk SFF enclosure should be split into two separate channels for maximum sequential IO bandwidth on relatively few disk, and single channel for situations desiring very good sequential bandwidth but also better random IO.

I am not aware of any SAN vendors offering the high-density enclosures for 2.5in drives, except for HP in the Storage Works 2000 MSA line. This may indicate a serious lack of appreciation (or even understanding) of the importance of performance over capacity.

Below is my suggestion for the number of disks per x4 SAS port depending on the expected random IO. For maximum sequential bandwidth with the fewest disks, use the Low column. The medium configuration is probably best for most scenarios. The heavy and OLTP models should only be employed after filling the PCI-E slots in the medium configuration.

Disks per x4 SASLowMediumHeavyOLTP

Dell PowerVault Disk Enclosures

The current Dell lineup for 6Gbps SAS are both 2U, the MD1200 for 12 3.5in disks and the MD1220 for 24 2.5in disks. The previous generation comprised the 3U MD1000 for 15 3.5in disks and the 2U MD1120 for 24 2.5in disks. The MD 12x0 supports SSD drives, but Dell SSD drives are still 3Gbps.

The MD1220 with 24 73GB 15K drives is priced at $11,049. The H800 RAID Controller is $649 with 512M and $799 with 1GB NV cache.

The following is from the Dell PowerVault MD1200 and PowerVault MD1220 Interoperability Guide.



In other diagrams, I had suggested attaching either
  1) one controller with both ports to a single enclosure, or
  2) one controller with each port to a separate enclosure.
The first option can be accomplished with the Single Host, Single Controller, Redundant Path connection, the second diagram in Figure 1.

The second option could be either one controller with each port connected to a stand-alone MD1220, or the Single Host, Single Controller Redundant Path shown below right with two MD1220 enclosures.