Repeat After Me: SATA Does Not Belong In Servers Part Deux

For the last number of years we have stopped deploying servers with SATA drives installed.

There are so many reasons why we stopped but here are a few comparisons to SCSI/SAS:

• SATA does not have the ability to manage a high I/O workload
• SATA only offers a single inbound and outbound data port while SAS offers dual ports for redundant paths
• SATA does not have the health monitoring capabilities with SMART certainly not cutting it
• SATA does not offer anywhere near the capabilities and command set that SAS does for server related tasks, disk redundancy, disk sharing, and so much more

There is a reason why disk manufacturers have tacked on SAS controllers to SATA platter sets. These so-called NearLine drives offer all of the SAS goodness but with SATA capacities.

Here is the first public, that I know of, presentation from Microsoft on the _why_ SATA does not belong in servers.

• Microsoft KB2801713: Hyper-V storage: Caching layers and implications for consistency

To quote specifically:

1.Use the per I/O control mechanism that is known as Force Unit Access (FUA). This flag specifies that the drive should write the data to stable media storage before signaling (sic) is finished. Applications that have to do this make sure that data is stable on the disk issue FUA to make sure that data is not lost if a power failure occurs.

Server-class disk drives (SCSI and Fibre Channel) generally support the FUA flag. On commodity drives (ATA, SATA, and USB), FUA might not be honored. (emphasis added) This can potentially leave data in an inconsistent state unless the drive's write cache is disabled. Make sure that the disk subsystem handles FUA correctly if you depend on this mechanism

When listening to a discussion on this the above applies even when SATA disks are used in a properly configured RAID setup whether software (host-based) or hardware RAID on Chip.

In addition, if one were to be setting up a Storage Spaces cluster with multiple paths to the JBOD unit then one would be required to set it up with SAS based SSDs for the high performance storage tier. SATA will work in a single server and single enclosure lab like setting but _not_ in production.

We have had other posts on this topic that outline many other reasons for our decision to drop SATA in servers. The SATA category and the SAS category would be one place to start. :)

Philip Elder
Microsoft MVP
MPECS Inc.
Co-Author: SBS 2008 Blueprint Book

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