In contrast, SATA drives have no ability to inform the host of a specific error that occurred during operation or to provide any information on how exactly the drive handles any issue.Īnother useful characteristic of the SAS protocol for data recovery is that reading sectors while ignoring ECC is supported for the entire capacity of the drive. The fact that some of these error-reporting messages can be provided asynchronously with the drive’s operation makes this system even more useful. Here are some examples of diagnostic messages: 'FAILURE PREDICTION THRESHOLD EXCEEDED', 'LOGICAL UNIT FAILED SELF-CONFIGURATION', 'MECHANICAL POSITIONING ERROR', 'DEFECT LIST ERROR' and the following are examples of data access error handling messages: 'READ RETRIES EXHAUSTED', 'RECOVERED DATA WITHOUT ECC - DATA AUTO-REALLOCATED', 'AUTO REALLOCATE FAILED', 'ADDRESS MARK NOT FOUND FOR ID FIELD'. This reporting system helps with the diagnostics of the drive by providing much better information on what exactly is happening behind the scenes and how the drive itself handles specific issues while accessing the data. This is clearly an important fact since it gives us the ability to validate the state of the drive, distinguishing “non-responding” situations from “currently processing” ones.Ī significant advantage of SAS drives from the data recovery perspective is that SAS/SCSI protocols have an extended error-reporting system providing more information on issues encountered by the drive. When a SATA drive is processing any initialization or read/write operation, its interface is locked with a busy status and so the drive is not even “listening” to any commands, while a SAS drive would still be communicating to the host and providing the current status of its operation, such as 'POWER ON OCCURRED', 'LOGICAL UNIT TRANSITIONING TO ANOTHER POWER CONDITION', 'LOGICAL UNIT IS IN PROCESS OF BECOMING READY', ' SELF-TEST IN PROGRESS', etc. The first advantage is that many commands are processed by the drive’s Micro Controller Unit (MCU) asynchronously to the drive’s operation. The good news is that the SAS protocol has some advantages when compared to ATA. That is one of the reasons why selection of a SAS controller used in your recovery processes is usually a crucial step. If something goes wrong during this lengthy process then the SAS controller may reject the connection and fail to even discover the device. The same task of requesting the status from a SAS drive requires multiple connection steps, such as going through many PHY negotiation states, discovery of the SAS device and its attributes like its SAS address, configuring SAS link and port to properly communicate to a discovered device, and only after that the host can actually send the first SAS command to a device to check its status. For example, when a SATA drive powers up, the host can check its status right away by reading its ATA Registers based on the controller’s port number that the drive is connected to. This means that connection to SAS drives with various instabilities is more difficult to control when compared to SATA drives. One of the critical challenges of the SAS protocol is that it has a connection state, while the ATA protocol is stateless. No adapters or converters can be used to connect a SAS drive to your SATA/IDE recovery tools because these two protocols have entirely different sets of commands, subsystems, and concepts used in their architecture, which results in different recovery methods applicable to SAS and SATA drives. SAS drives cannot be accommodated by recovery processes built for the ATA protocol. The first thing to note is that even though SAS and SATA have a similar physical level of the interface, their protocols have nothing in common. In this post, we would like to cover the most critical aspects related to recovery processes of SAS drives and how they compare to SATA devices. As SAS drives become more popular on the market, data recovery professionals are facing new challenges in handling various issues introduced by these devices.
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