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docs: add blkdebug block driver documentation
The blkdebug block driver is undocumented. Documenting it is worthwhile since it offers powerful error injection features that are used by qemu-iotests test cases. This document will make it easier for people to learn about and use blkdebug. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Stefan Hajnoczi
Kevin Wolf
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docs/blkdebug.txt
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docs/blkdebug.txt
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Block I/O error injection using blkdebug
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----------------------------------------
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Copyright (C) 2014 Red Hat Inc
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This work is licensed under the terms of the GNU GPL, version 2 or later. See
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the COPYING file in the top-level directory.
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The blkdebug block driver is a rule-based error injection engine. It can be
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used to exercise error code paths in block drivers including ENOSPC (out of
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space) and EIO.
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This document gives an overview of the features available in blkdebug.
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Background
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----------
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Block drivers have many error code paths that handle I/O errors. Image formats
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are especially complex since metadata I/O errors during cluster allocation or
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while updating tables happen halfway through request processing and require
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discipline to keep image files consistent.
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Error injection allows test cases to trigger I/O errors at specific points.
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This way, all error paths can be tested to make sure they are correct.
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Rules
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-----
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The blkdebug block driver takes a list of "rules" that tell the error injection
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engine when to fail an I/O request.
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Each I/O request is evaluated against the rules. If a rule matches the request
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then its "action" is executed.
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Rules can be placed in a configuration file; the configuration file
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follows the same .ini-like format used by QEMU's -readconfig option, and
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each section of the file represents a rule.
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The following configuration file defines a single rule:
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$ cat blkdebug.conf
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[inject-error]
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event = "read_aio"
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errno = "28"
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This rule fails all aio read requests with ENOSPC (28). Note that the errno
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value depends on the host. On Linux, see
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/usr/include/asm-generic/errno-base.h for errno values.
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Invoke QEMU as follows:
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$ qemu-system-x86_64
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-drive if=none,cache=none,file=blkdebug:blkdebug.conf:test.img,id=drive0 \
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-device virtio-blk-pci,drive=drive0,id=virtio-blk-pci0
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Rules support the following attributes:
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event - which type of operation to match (e.g. read_aio, write_aio,
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flush_to_os, flush_to_disk). See the "Events" section for
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information on events.
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state - (optional) the engine must be in this state number in order for this
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rule to match. See the "State transitions" section for information
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on states.
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errno - the numeric errno value to return when a request matches this rule.
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The errno values depend on the host since the numeric values are not
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standarized in the POSIX specification.
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sector - (optional) a sector number that the request must overlap in order to
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match this rule
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once - (optional, default "off") only execute this action on the first
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matching request
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immediately - (optional, default "off") return a NULL BlockDriverAIOCB
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pointer and fail without an errno instead. This exercises the
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code path where BlockDriverAIOCB fails and the caller's
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BlockDriverCompletionFunc is not invoked.
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Events
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------
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Block drivers provide information about the type of I/O request they are about
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to make so rules can match specific types of requests. For example, the qcow2
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block driver tells blkdebug when it accesses the L1 table so rules can match
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only L1 table accesses and not other metadata or guest data requests.
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The core events are:
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read_aio - guest data read
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write_aio - guest data write
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flush_to_os - write out unwritten block driver state (e.g. cached metadata)
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flush_to_disk - flush the host block device's disk cache
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See block/blkdebug.c:event_names[] for the full list of events. You may need
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to grep block driver source code to understand the meaning of specific events.
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State transitions
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-----------------
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There are cases where more power is needed to match a particular I/O request in
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a longer sequence of requests. For example:
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write_aio
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flush_to_disk
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write_aio
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How do we match the 2nd write_aio but not the first? This is where state
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transitions come in.
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The error injection engine has an integer called the "state" that always starts
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initialized to 1. The state integer is internal to blkdebug and cannot be
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observed from outside but rules can interact with it for powerful matching
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behavior.
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Rules can be conditional on the current state and they can transition to a new
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state.
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When a rule's "state" attribute is non-zero then the current state must equal
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the attribute in order for the rule to match.
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For example, to match the 2nd write_aio:
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[set-state]
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event = "write_aio"
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state = "1"
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new_state = "2"
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[inject-error]
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event = "write_aio"
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state = "2"
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errno = "5"
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The first write_aio request matches the set-state rule and transitions from
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state 1 to state 2. Once state 2 has been entered, the set-state rule no
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longer matches since it requires state 1. But the inject-error rule now
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matches the next write_aio request and injects EIO (5).
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State transition rules support the following attributes:
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event - which type of operation to match (e.g. read_aio, write_aio,
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flush_to_os, flush_to_disk). See the "Events" section for
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information on events.
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state - (optional) the engine must be in this state number in order for this
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rule to match
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new_state - transition to this state number
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Suspend and resume
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------------------
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Exercising code paths in block drivers may require specific ordering amongst
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concurrent requests. The "breakpoint" feature allows requests to be halted on
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a blkdebug event and resumed later. This makes it possible to achieve
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deterministic ordering when multiple requests are in flight.
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Breakpoints on blkdebug events are associated with a user-defined "tag" string.
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This tag serves as an identifier by which the request can be resumed at a later
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point.
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See the qemu-io(1) break, resume, remove_break, and wait_break commands for
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details.
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