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diff --git a/docs/blkdebug.txt b/docs/blkdebug.txt new file mode 100644 index 0000000..5dde072 --- /dev/null +++ b/docs/blkdebug.txt @@ -0,0 +1,161 @@ +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. |