Chapter 4. Understanding The Medium Errors And Bad Blocks - IBM Storwize V7000 Troubleshooting And Maintenance Manual

Chapter 4. Understanding the medium errors and bad blocks

A storage system returns a medium error response to a host when it is unable to
successfully read a block. The Storwize V7000 response to a host read follows this
behavior.
The volume virtualization that is provided extends the time when a medium error
is returned to a host. Because of this difference to non-virtualized systems, the
Storwize V7000 uses the term bad blocks rather than medium errors.
The Storwize V7000 allocates volumes from the extents that are on the managed
disks (MDisks). The MDisk can be a volume on an external storage controller or a
RAID array that is created from internal drives. In either case, depending on the
RAID level used, there is normally protection against a read error on a single
drive. However, it is still possible to get a medium error on a read request if
multiple drives have errors or if the drives are rebuilding or are offline due to
other issues.
The Storwize V7000 provides migration facilities to move a volume from one
underlying set of physical storage to another or to replicate a volume that uses
FlashCopy or Metro Mirror or Global Mirror. In all these cases, the migrated
volume or the replicated volume returns a medium error to the host when the
logical block address on the original volume is read. The system maintains tables
of bad blocks to record where the logical block addresses that cannot be read are.
These tables are associated with the MDisks that are providing storage for the
volumes.
The dumpmdiskbadblocks command and the dumpallmdiskbadblocks command are
available to query the location of bad blocks.
Important: The dumpmdiskbadblocks only outputs the virtual medium errors that
have been created, and not a list of the actual medium errors on MDisks or drives.
It is possible that the tables that are used to record bad block locations can fill up.
The table can fill either on an MDisk or on the system as a whole. If a table does
fill up, the migration or replication that was creating the bad block fails because it
was not possible to create an exact image of the source volume.
The system creates alerts in the event log for the following situations:
v When it detects medium errors and creates a bad block
v When the bad block tables fill up
The following errors are identified:
© Copyright IBM Corp. 2010, 2013
Table 18. Bad block errors
Error code
1840
1226
Description
The managed disk has bad blocks. On an
external controller, this can only be a copied
medium error.
The system has failed to create a bad block
because the MDisk already has the
maximum number of allowed bad blocks.
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