Understanding SCSI Sense

This page is about decoding and interpreting the SCSI sense buffer in order to troubleshoot a disk or storage device problem.

A SCSI sense buffer is the error reporting facility in SCSI. It reports the error code and possibly also additional information that helps to locate the source of the problem so the administrator or developer can help resolve the issue.

A SCSI sense has several top-level attributes that one would care about the most:

• Sense type, either fixed or descriptor,
• What command it relates to, current or previous,
• Sense Key,

The easiest way to decode a sense buffer is to use a tool, I know of two:

The explanation below would focus a bit on how to decode and also what can be understood from it.

The sense type is important to decode the sense buffer, you need to know if it is a fixed format or a descriptor format. The most common format is the fixed format and most of the direct decoding instruction below will be about the fixed format the descriptor format is more complex and less frequent but it’s worth being aware of its existence. The details both formats provide are the same just the decoding mechanics are different.

One important distinction about a sense buffer is wether the sense is about the command that failed with the sense or a previous command. It is entirely possible that the command that returned with an error is not at all at fault and that everything is just fine with it but that a previous command that was already acknowledged went bad at the end and the SCSI target has no other way to tell the user about the problem. In such a case some random other command will be failed with a sense buffer that indicates the problem was in a previous command.

The first byte to look at is byte 0, and what matters there are the 7 lower bits, so if the number is at or above 80h (128 decimal) you need to substruct 80h to get the actual value. There are only 4 permitted values for these 7 lower bits:

• 70h — fixed format, current sense
• 71h — fixed format, previous sense
• 72h — descriptor format, current sense
• 73h — descriptor format, previous sense

The next important information is in byte 2 (number 3 if counting from 1), the four lowest bits are the sense key, since the sense buffer is given in hexadecimal numbers this is the second character of the number. The sense key is the key to understand the error code. It tells you the high level issue and it is detailed below with their meanings.

The next part is the ASC and ASCQ these are found in bytes 12 and 13 (13 and 14 if counting from 1). These explain in somewhat more detail the specifics of the problem.

Take a look at the following example:

f1 00 03 02 DD 7E BF 18 00 00 00 00 0C 03 00 00 00 00 00 00 03 0C 03 00 00 0F 83 01 00 08 00 00


The first byte is F1h, we remember to remove the top bit and we get 71h which means “fixed format, previous sense” so we can further decode it according our instructions and remember that the IO that failed with this sense is not to blame, it was a previous IO that failed. Next up we find 3h as the second nibble in the third byte which tells us that this is a medium error. A disk tried to read or write and failed. The last part is the ASC and ASCQ which are 0Ch/03h and this translates to “WRITE ERROR – RECOMMEND REASSIGNMENT”. This tells us it was a write that failed and that the disk is suggesting to reassign the sector. One part that is a bit harder to decode is what is the LBA that actually failed. The first bit of the first byte that is lighted says that the information field has meaning and in the case of a medium error (sense key 3h) the meaning is the first LBA that failed.

You can see full parsing of this sense in the webapp: f1 00 03 02 DD 7E BF 18 00 00 00 00 0C 03 00 00 00 00 00 00 03 0C 03 00 00 0F 83 01 00 08 00 00

The sense keys are listed briefly at the T10 Sense Key page. The ASC/ASCQ are listed at the T10 ASC/ASCQ page.

Common sense keys are:

• 1h Recovered Error — informational only
• 2h Not ready — temporary error, need to wait it out
• 3h Medium Error — may work if retried, disappears after write or reassign
• 4h Hardware Error — usually permanent failure
• 5h Illegal Request — mostly a programming error, maybe the device handles an older standard with some bits unsupported
• 6h Unit Attention — a storage fabric problem, usually a notification and not a problem with the IO itself
• 7h Data Protect — The device cannot be read/written, needs to be unlocked (physically or logically)
• Bh Aborted Task — Fabric problem, command may be retried but possibly a bad cable

Recovered Error

A recovered error is the least problematic in one way since it only says that there was a problem that the storage device managed to take care of and is just letting the user know about in case he is curious or would like to delve deeper and find what is going on.

There are two reasons why a recovered error would be returned:

• SMART Trip
• Medium Errors recovered

If a disk finds that it is about to fail according to its SMART logic (also known as Informational Exceptions in SCSI), it will report it in log page 2Fh but the only way for it to tell the storage system that now is the time to start looking at this log page is by taking one random IO (the first one that comes up after the SMART issue is detected) and return a correctable error with ASC/ASCQ of 5Dh/00h which stands for “FAILURE PREDICTION THRESHOLD EXCEEDED”.

If a sector is having problems and it took a non-trivial amount of work to recover from it a recovered error may be reported with an ASC of 11h, 17h or 18h depending on the severity and the type of recovery needed.

If a specific device will or will not return a recovered error sense is determined by some parameters in the mode pages. You may want to peruse them to find how to turn on or off this behavior.

The normal Linux kernel SCSI stack will ignore this sense and continue along with only reporting it in dmesg.

Device not ready, wait and retry, device is either going to get good or fail and it should timeout itself to Hardware Error if so.

A Not Ready sense is returned when the device is powering up and not yet ready to really respond to anything serious, such as when an HDD is still spinning up or when an SSD has still not read its metadata tables from the flash.

Under some error conditions this may persist for some while and if it persists for more than 30 seconds or so it is likely to be a failure already. In most cases the device will have a timeout of its own after which it will transition to replying 4h Hardware Error instead of the Not Ready reply.

A user can only wait a bit more for the device to get ready and fail it out if it takes too long to exit this state.

Medium Error

A medium error means that you tried to read or write data and the disk failed. It also is taken to mean that the problem is not permanent and doesn’t afflict the entire disk only some area of it. A disk can reassign the affected to solve the problem. If the disk is configured to auto-reassign than a write to that area will cause the disk to reassign and the problem will be gone, if the disk is not configured to auto-reassign then you need to use the REASSIGN BLOCKS command to get that same effect.

At some cases a retry to read the data may get the data eventually but it doesn’t have a high likelyhood and it incurs a great penalty in time since normally a medium error is declared after a timeout is reached during the read operation.

In a SCSI disk there are two bits AWRE and ARRE that control if a auto-reallocation is done by the disk.

Hardware Error

An hardware error is reported when the disk reaches a fatal state and will not recover from this. The disk can no longer be read or written. Not even a power cycle will help in this case.

Illegal Request

When a disk returns “Illegal Request” it means it failed to parse the command or the data you gave it. Either the command is invalid or it is unsupported by the disk. This can happen when the disk supports an older standard or doesn’t adhere to the standard completely.

When doing MODE SELECT and LOG SELECT commands if the parameter you are trying to change is unsupported for change you will also get an Illegal Request. You can get the Changeable Mask for MODE SELECT with MODE SENSE to see if this is the case.

You will need to reformulate the request or plain avoid it altogether.

Unit Attention

A Unit Attention is the way for the device to tell you that it’s operational state or the fabric state has changed. Since SCSI is a client-server protocol there is no other way for the device to tell you that something changed without piggy-backing on another request which is exactly what happens here. The command that you performed is likely to be just fine but there was some other condition in the device that requires the user’s attention. The attention needs to be taken care of and the command that was unfortunate enough to be failed for this can be retried.

Examples for this can be when MODE SELECT is used to set Mode Parameters, when an initiator is lost and then you get an I_T_L NEXUS LOSS or several other such cases.

Data Protect

Data Protect is received when the device is working but locked, either a physical write lock or for Data-at-Rest encryption when the device was not yet unlocked or the band was not yet unlocked.

This only means that unlock needs to happen for the action to be allowed.