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Many tape devices will automatically rewind following a poweron/reset. This can result in data loss as other operations in the driver can write to the tape when the position is unknown. E.g. MTEOM can write a filemark at the beginning of the tape. This patch adds code to detect poweron/reset unit attentions and prevents the driver from writing to the tape when the position could be unknown. Customer reported problem description: We have experienced an issue with the SCSI tape driver (st) which has led to data loss for us on two separate occasions in production, as well as in a third case in which we were able to reproduce the failure in our test environment. The tape device involved is an Amazon Tape Gateway, a virtual tape library (VTL) appliance which presents as a series of iSCSI targets (multiple tape drives and a changer) and is backed by storage in Amazon S3. The problem is a general one and not limited to any particular SCSI transport or tape device, though the nature of both iSCSI and the VTL make data loss somewhat more likely with this combination than with a physical tape drive. The observed behavior occurs when an error causes the VTL tape gateway process (on the appliance) to crash and restart. This interrupts the iSCSI TCP connections and, when it occurs during a write, causes the write to fail with EIO. However, we then find that the virtual tape in question is now completely blank. We raised this issue with AWS support, thinking this must be a bug in the VTL appliance, but that turns out not to be the case. Per AWS support, when the gateway crashes in this manner, its notion of the current tape position is reset to the beginning of the tape. It also sets a unit attention condition, such that the next request results in a CHECK CONDITION status with sense key UNIT ATTENTION and asc/ascq indicating a device reset. According to their logs the next command being sent is WRITE FILEMARK, which results in writing an FM at the beginning of the tape, effectively discarding its contents. In fact, once the write fails with EIO, our software attempts to recover by rewinding and repositioning the tape, then resuming operation. If this fails, it attempts to rewind and reposition again, write a marker at the end of the tape, and then unmount. It does not under any circumstances write either data or filemarks without having successfully positioned the tape to a known point. What actually happens is that, since the last operation was a write, the kernel executes an implied MTWEOF operation (which translate to a Write Filemarks command) before the rewind that was actually requested. This seems not entirely unreasonable, provided the tape position is known. However, once this request fails (due to the unit attention condition), our next rewind attempt also triggers an implied MTWEOF, which does _not_ fail (the unit attention condition persists only until the initiator has been notified); this is the command that unexpectedly erases the tape. Our analysis is that the st driver is in fact completely ignoring the UNIT ATTENTION and associated reset notification from the device. This is not a condition that can be detected in the transport or mid-layer, as it occurs entirely within the target and is reported only via the UNIT ATTENTION sense key. The upper driver (i.e. st) needs to detect this indication and reset its internal model of the device to an unknown state. Suggested-by: Jeffrey Hutzelman <jhutz@cmu.edu> Signed-off-by: John Meneghini <jmeneghi@redhat.com> Link: https://lore.kernel.org/r/20230822181413.1210647-1-jmeneghi@redhat.com Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.