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authorStefan Hajnoczi <stefanha@redhat.com>2014-09-24 10:44:14 +0100
committerKevin Wolf <kwolf@redhat.com>2014-09-25 15:24:14 +0200
commitf9d7b4b3b4249336be665180a1aea7ce5497d2ca (patch)
tree4d707e6bc21392ef6473b49827cf7d858c0ca635 /docs/blkdebug.txt
parent5abbf0ee4d87c695deb1c3fca9bb994b93a3e3be (diff)
downloadqemu-arm-f9d7b4b3b4249336be665180a1aea7ce5497d2ca.tar.gz
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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+Block I/O error injection using blkdebug
+----------------------------------------
+Copyright (C) 2014 Red Hat Inc
+
+This work is licensed under the terms of the GNU GPL, version 2 or later. See
+the COPYING file in the top-level directory.
+
+The blkdebug block driver is a rule-based error injection engine. It can be
+used to exercise error code paths in block drivers including ENOSPC (out of
+space) and EIO.
+
+This document gives an overview of the features available in blkdebug.
+
+Background
+----------
+Block drivers have many error code paths that handle I/O errors. Image formats
+are especially complex since metadata I/O errors during cluster allocation or
+while updating tables happen halfway through request processing and require
+discipline to keep image files consistent.
+
+Error injection allows test cases to trigger I/O errors at specific points.
+This way, all error paths can be tested to make sure they are correct.
+
+Rules
+-----
+The blkdebug block driver takes a list of "rules" that tell the error injection
+engine when to fail an I/O request.
+
+Each I/O request is evaluated against the rules. If a rule matches the request
+then its "action" is executed.
+
+Rules can be placed in a configuration file; the configuration file
+follows the same .ini-like format used by QEMU's -readconfig option, and
+each section of the file represents a rule.
+
+The following configuration file defines a single rule:
+
+ $ cat blkdebug.conf
+ [inject-error]
+ event = "read_aio"
+ errno = "28"
+
+This rule fails all aio read requests with ENOSPC (28). Note that the errno
+value depends on the host. On Linux, see
+/usr/include/asm-generic/errno-base.h for errno values.
+
+Invoke QEMU as follows:
+
+ $ qemu-system-x86_64
+ -drive if=none,cache=none,file=blkdebug:blkdebug.conf:test.img,id=drive0 \
+ -device virtio-blk-pci,drive=drive0,id=virtio-blk-pci0
+
+Rules support the following attributes:
+
+ event - which type of operation to match (e.g. read_aio, write_aio,
+ flush_to_os, flush_to_disk). See the "Events" section for
+ information on events.
+
+ state - (optional) the engine must be in this state number in order for this
+ rule to match. See the "State transitions" section for information
+ on states.
+
+ errno - the numeric errno value to return when a request matches this rule.
+ The errno values depend on the host since the numeric values are not
+ standarized in the POSIX specification.
+
+ sector - (optional) a sector number that the request must overlap in order to
+ match this rule
+
+ once - (optional, default "off") only execute this action on the first
+ matching request
+
+ immediately - (optional, default "off") return a NULL BlockDriverAIOCB
+ pointer and fail without an errno instead. This exercises the
+ code path where BlockDriverAIOCB fails and the caller's
+ BlockDriverCompletionFunc is not invoked.
+
+Events
+------
+Block drivers provide information about the type of I/O request they are about
+to make so rules can match specific types of requests. For example, the qcow2
+block driver tells blkdebug when it accesses the L1 table so rules can match
+only L1 table accesses and not other metadata or guest data requests.
+
+The core events are:
+
+ read_aio - guest data read
+
+ write_aio - guest data write
+
+ flush_to_os - write out unwritten block driver state (e.g. cached metadata)
+
+ flush_to_disk - flush the host block device's disk cache
+
+See block/blkdebug.c:event_names[] for the full list of events. You may need
+to grep block driver source code to understand the meaning of specific events.
+
+State transitions
+-----------------
+There are cases where more power is needed to match a particular I/O request in
+a longer sequence of requests. For example:
+
+ write_aio
+ flush_to_disk
+ write_aio
+
+How do we match the 2nd write_aio but not the first? This is where state
+transitions come in.
+
+The error injection engine has an integer called the "state" that always starts
+initialized to 1. The state integer is internal to blkdebug and cannot be
+observed from outside but rules can interact with it for powerful matching
+behavior.
+
+Rules can be conditional on the current state and they can transition to a new
+state.
+
+When a rule's "state" attribute is non-zero then the current state must equal
+the attribute in order for the rule to match.
+
+For example, to match the 2nd write_aio:
+
+ [set-state]
+ event = "write_aio"
+ state = "1"
+ new_state = "2"
+
+ [inject-error]
+ event = "write_aio"
+ state = "2"
+ errno = "5"
+
+The first write_aio request matches the set-state rule and transitions from
+state 1 to state 2. Once state 2 has been entered, the set-state rule no
+longer matches since it requires state 1. But the inject-error rule now
+matches the next write_aio request and injects EIO (5).
+
+State transition rules support the following attributes:
+
+ event - which type of operation to match (e.g. read_aio, write_aio,
+ flush_to_os, flush_to_disk). See the "Events" section for
+ information on events.
+
+ state - (optional) the engine must be in this state number in order for this
+ rule to match
+
+ new_state - transition to this state number
+
+Suspend and resume
+------------------
+Exercising code paths in block drivers may require specific ordering amongst
+concurrent requests. The "breakpoint" feature allows requests to be halted on
+a blkdebug event and resumed later. This makes it possible to achieve
+deterministic ordering when multiple requests are in flight.
+
+Breakpoints on blkdebug events are associated with a user-defined "tag" string.
+This tag serves as an identifier by which the request can be resumed at a later
+point.
+
+See the qemu-io(1) break, resume, remove_break, and wait_break commands for
+details.