Redis從入門到放棄系列(六) 持久化

Redis從入門到放棄系列(六) 持久化

本文例子基於：5.0.4 Redis持久化機制包括兩種,RDB和AOF,當兩種機制都存在的時候,redis啓動時會採用aof來恢復數據

Redis爲了保證數據的持久性,提供了RDB跟AOF機制,RDB是內存數據的二進制序列化形式,在存儲上很是緊湊,AOF日誌記錄的是數據修改的指令記錄文本。

RDB

RDB機制其具體實現爲使用系統的多進程寫時複製(Copy On Write)，寫時複製是一種能夠推遲甚至避免拷貝數據的技術。內核此時並不複製整個進程的地址空間，而是讓父子進程共享同一個地址空間。只用在須要寫入的時候纔會複製地址空間，從而使各個進行擁有各自的地址空間。

咱們知道redis是單線程的,當redis在接受請求的時候還要處理rdb save,若是數據量很大,那麼對於業務的影響是致命的,因此redis採用了多進程寫時複製技術來解決該問題.父進程首先fork一個子進程，RDB持久化交給了子進程來進行,父進程繼續處理客戶端的請求.

那麼redis在何時會觸發RDB持久化呢? 咱們來看一下redis.conf裏面關於RDB持久化的說明：

################################ SNAPSHOTTING  ################################
#
# Save the DB on disk:
#
#   save <seconds> <changes>
#
#   Will save the DB if both the given number of seconds and the given
#   number of write operations against the DB occurred.
#
#   In the example below the behaviour will be to save:
#   after 900 sec (15 min) if at least 1 key changed
#   after 300 sec (5 min) if at least 10 keys changed
#   after 60 sec if at least 10000 keys changed
#
#   Note: you can disable saving completely by commenting out all "save" lines.
#
#   It is also possible to remove all the previously configured save
#   points by adding a save directive with a single empty string argument
#   like in the following example:
#
#   save ""

save 900 1
save 300 10
save 60 10000

# By default Redis will stop accepting writes if RDB snapshots are enabled
# (at least one save point) and the latest background save failed.
# This will make the user aware (in a hard way) that data is not persisting
# on disk properly, otherwise chances are that no one will notice and some
# disaster will happen.
#
# If the background saving process will start working again Redis will
# automatically allow writes again.
#
# However if you have setup your proper monitoring of the Redis server
# and persistence, you may want to disable this feature so that Redis will
# continue to work as usual even if there are problems with disk,
# permissions, and so forth.
stop-writes-on-bgsave-error yes

# Compress string objects using LZF when dump .rdb databases?
# For default that's set to 'yes' as it's almost always a win.
# If you want to save some CPU in the saving child set it to 'no' but
# the dataset will likely be bigger if you have compressible values or keys.
rdbcompression yes

# Since version 5 of RDB a CRC64 checksum is placed at the end of the file.
# This makes the format more resistant to corruption but there is a performance
# hit to pay (around 10%) when saving and loading RDB files, so you can disable it
# for maximum performances.
#
# RDB files created with checksum disabled have a checksum of zero that will
# tell the loading code to skip the check.
rdbchecksum yes

# The filename where to dump the DB
dbfilename dump.rdb

# The working directory.
#
# The DB will be written inside this directory, with the filename specified
# above using the 'dbfilename' configuration directive.
#
# The Append Only File will also be created inside this directory.
#
# Note that you must specify a directory here, not a file name.
dir ./

咱們能夠看到，redis默認有3個措施觸發RDB的持久化：

• 60秒有10000個key改變
• 300秒有10個key改變
• 900秒有1個key改變 當以上任意條件知足則觸發RDB持久化機制.若是不須要RDB持久化,能夠把該配置註釋掉.RDB最嚴重的狀況下會存在redis佔據原先數據內存的2倍大小。 RDB默認啓用LZF壓縮字符串對象,以節省儲存空間。

AOF

讓咱們先來了解一下redis.conf裏面對於aof持久化的配置說明:

############################## APPEND ONLY MODE ###############################

# By default Redis asynchronously dumps the dataset on disk. This mode is
# good enough in many applications, but an issue with the Redis process or
# a power outage may result into a few minutes of writes lost (depending on
# the configured save points).
#
# The Append Only File is an alternative persistence mode that provides
# much better durability. For instance using the default data fsync policy
# (see later in the config file) Redis can lose just one second of writes in a
# dramatic event like a server power outage, or a single write if something
# wrong with the Redis process itself happens, but the operating system is
# still running correctly.
#
# AOF and RDB persistence can be enabled at the same time without problems.
# If the AOF is enabled on startup Redis will load the AOF, that is the file
# with the better durability guarantees.
#
# Please check http://redis.io/topics/persistence for more information.

appendonly no

# The name of the append only file (default: "appendonly.aof")

appendfilename "appendonly.aof"

# The fsync() call tells the Operating System to actually write data on disk
# instead of waiting for more data in the output buffer. Some OS will really flush
# data on disk, some other OS will just try to do it ASAP.
#
# Redis supports three different modes:
#
# no: don't fsync, just let the OS flush the data when it wants. Faster.
# always: fsync after every write to the append only log. Slow, Safest.
# everysec: fsync only one time every second. Compromise.
#
# The default is "everysec", as that's usually the right compromise between
# speed and data safety. It's up to you to understand if you can relax this to
# "no" that will let the operating system flush the output buffer when
# it wants, for better performances (but if you can live with the idea of
# some data loss consider the default persistence mode that's snapshotting),
# or on the contrary, use "always" that's very slow but a bit safer than
# everysec.
#
# More details please check the following article:
# http://antirez.com/post/redis-persistence-demystified.html
#
# If unsure, use "everysec".

# appendfsync always
appendfsync everysec
# appendfsync no

# When the AOF fsync policy is set to always or everysec, and a background
# saving process (a background save or AOF log background rewriting) is
# performing a lot of I/O against the disk, in some Linux configurations
# Redis may block too long on the fsync() call. Note that there is no fix for
# this currently, as even performing fsync in a different thread will block
# our synchronous write(2) call.
#
# In order to mitigate this problem it's possible to use the following option
# that will prevent fsync() from being called in the main process while a
# BGSAVE or BGREWRITEAOF is in progress.
#
# This means that while another child is saving, the durability of Redis is
# the same as "appendfsync none". In practical terms, this means that it is
# possible to lose up to 30 seconds of log in the worst scenario (with the
# default Linux settings).
#
# If you have latency problems turn this to "yes". Otherwise leave it as
# "no" that is the safest pick from the point of view of durability.

no-appendfsync-on-rewrite no

# Automatic rewrite of the append only file.
# Redis is able to automatically rewrite the log file implicitly calling
# BGREWRITEAOF when the AOF log size grows by the specified percentage.
#
# This is how it works: Redis remembers the size of the AOF file after the
# latest rewrite (if no rewrite has happened since the restart, the size of
# the AOF at startup is used).
#
# This base size is compared to the current size. If the current size is
# bigger than the specified percentage, the rewrite is triggered. Also
# you need to specify a minimal size for the AOF file to be rewritten, this
# is useful to avoid rewriting the AOF file even if the percentage increase
# is reached but it is still pretty small.
#
# Specify a percentage of zero in order to disable the automatic AOF
# rewrite feature.

auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb

# An AOF file may be found to be truncated at the end during the Redis
# startup process, when the AOF data gets loaded back into memory.
# This may happen when the system where Redis is running
# crashes, especially when an ext4 filesystem is mounted without the
# data=ordered option (however this can't happen when Redis itself
# crashes or aborts but the operating system still works correctly).
#
# Redis can either exit with an error when this happens, or load as much
# data as possible (the default now) and start if the AOF file is found
# to be truncated at the end. The following option controls this behavior.
#
# If aof-load-truncated is set to yes, a truncated AOF file is loaded and
# the Redis server starts emitting a log to inform the user of the event.
# Otherwise if the option is set to no, the server aborts with an error
# and refuses to start. When the option is set to no, the user requires
# to fix the AOF file using the "redis-check-aof" utility before to restart
# the server.
#
# Note that if the AOF file will be found to be corrupted in the middle
# the server will still exit with an error. This option only applies when
# Redis will try to read more data from the AOF file but not enough bytes
# will be found.
aof-load-truncated yes

# When rewriting the AOF file, Redis is able to use an RDB preamble in the
# AOF file for faster rewrites and recoveries. When this option is turned
# on the rewritten AOF file is composed of two different stanzas:
#
#   [RDB file][AOF tail]
#
# When loading Redis recognizes that the AOF file starts with the "REDIS"
# string and loads the prefixed RDB file, and continues loading the AOF
# tail.
aof-use-rdb-preamble yes

AOF日誌記錄了服務器接收到的每一次寫入操做，這些操做會在redis服務器啓動的時候從新執行一遍,重構原始數據集。經過只能追加的方式,使用與redis協議相同的格式記錄。當aof日誌文件過大的時候，會觸發aof-rewriete。

觸發aof持久化有三種策略:

• appendfsync always(當有寫入操做就觸發一次持久化)
• appendfsync everysec(每秒持久化一次,生產使用)
• appendfsync no(不進行同步,讓操做系統決定在須要的時候將數據持久化)

當aof日誌文件過大時會在後臺自動重寫它。aof-rewrite是安全的。首先redis會開啓一個子進程對內存進行遍歷轉換成對應的redis的操做指令,將對應的操做指令序列化到一個新的aof日誌文件中。當序列化完成以後,redis會將序列化期間產生的aof變化同步到該aof文件中，當同步完畢以後替代本來舊的aof文件.

觸發aof-rewrite配置: - no-appendfsync-on-rewrite(重寫時是否能夠運行Appendfsync) - auto-aof-rewrite-percentage(當文件大小是上次rewrite後的大小的百分比) - auto-aof-rewrite-min-size(當aof文件不小於該數值)

在redis4.0以後啓用了混合持久化. 前一部分是RDB格式,後一部分是AOF,即在持久化的時候,接受客戶端請求過來的另外的緩存數據~

aof-use-rdb-preamble yes