Confirming end-to-end data consistency checking

6.10 LPAR mode z/VM guest

There are different types of end-to-end data consistency checking, with dependencies on hardware and software.

About this task

End-to-end data consistency checking is based on a data integrity field (DIF) that is added to transferred data blocks. DIF data is used to confirm that a data block originates from the expected source and was not modified during the transfer between the storage system and the FCP device. The SCSI Block Commands (T10 SBC) standard defines several types of DIF. Linux® data integrity extension (DIX) builds on DIF to extend consistency checking, for example, to the operating system, middleware, or an application.

The zfcp device driver supports the following modes of end-to-end data consistency checking:
  • The FCP device calculates and checks a DIF checksum (DIF type 1)

    Enable this mode with the zfcp.dif= kernel or dif= module parameter.

    Interface change: As of kernel 5.0, the zfcp.dif= kernel parameter enables DIF only and no longer includes DIX. Use the zfcp.dix= kernel parameter to enable end-to-end data consistency checking in extended mode, with both DIF and DIX.
  • The Linux block integrity layer calculates and checks a TCP/IP checksum, which the FCP device then translates to a DIF checksum (DIX type 1 with DIF type 1)

    Enable this mode with the zfcp.dix= kernel or dix= module parameter.

    Note: End-to-end data consistency checking in extended mode is experimental. SCSI disks for which this mode is enabled must be accessed with direct I/O. Direct I/O requires direct access through the block device or through a file system that fully supports end-to-end data consistency checking in extended mode. For example, XFS provides this support. Expect error messages about invalid checksums when you use other access methods.

For more information about the kernel or module parameters that control the end-to-end data consistency checking, see Setting up the zfcp device driver.

With end-to-end data consistency checking for SCSI disks enabled, Linux automatically discovers which FCP devices and which SCSI disks support end-to-end data consistency checking. No further setup is required.

For SCSI disks for which end-to-end data consistency checking is used, there is a sysfs directory 
/sys/block/sd<x>/integrity
In the path, sd<x> is the standard name of the block device of the SCSI disk.
End-to-end data consistency checking is used only if all of the following components support it:
SCSI disk
Check your storage server documentation about T10 DIF support and any restrictions.
IBM Z® hardware
IBM Z FCP adapter hardware supports end-to-end data consistency checking as of FICON® Express8.
Hypervisor
For Linux on z/VM®, you require a z/VM version with guest support for end-to-end data consistency checking.
FCP device
Check your FCP adapter hardware documentation about the support and any restrictions. For example, end-to-end data consistency checking might be supported only for disks with 512-byte block size.
Read the prot_capabilities sysfs attribute of the SCSI host that is associated with an FCP device to find out about its end-to-end data consistency checking support. The following values are possible:
0
The FCP device does not support end-to-end data consistency checking.
1
The FCP device supports DIF type 1.
17
The FCP device supports DIX type 1 with DIF type 1.

Procedure

Issue a command of this form: 
# cat /sys/bus/ccw/devices/<device_bus_id>/host<n>/scsi_host/host<n>/prot_capabilities
where <device_bus_id> identifies the FCP device and <n> is an integer that identifies the corresponding SCSI host.

Example

# cat /sys/bus/ccw/devices/0.0.1940/host0/scsi_host/host0/prot_capabilities
1