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How Redis writes data to disk
Persistence refers to the writing of data to durable storage, such as a solid-state disk (SSD). Redis provides a range of persistence options. These include:
If you'd rather not think about the tradeoffs between these different persistence strategies, you may want to consider Redis Enterprise's persistence options, which can be pre-configured using a UI.
To learn more about how to evaluate your Redis persistence strategy, read on.
FLUSHALL command, as long as no rewrite of the log was performed in the meantime, you can still save your data set just by stopping the server, removing the latest command, and restarting Redis again.Redis < 7.0
The general indication you should use both persistence methods is if you want a degree of data safety comparable to what PostgreSQL can provide you.
If you care a lot about your data, but still can live with a few minutes of data loss in case of disasters, you can simply use RDB alone.
There are many users using AOF alone, but we discourage it since to have an RDB snapshot from time to time is a great idea for doing database backups, for faster restarts, and in the event of bugs in the AOF engine.
The following sections will illustrate a few more details about the two persistence models.
By default Redis saves snapshots of the dataset on disk, in a binary
file called dump.rdb. You can configure Redis to have it save the
dataset every N seconds if there are at least M changes in the dataset,
or you can manually call the SAVE or BGSAVE commands.
For example, this configuration will make Redis automatically dump the dataset to disk every 60 seconds if at least 1000 keys changed:
save 60 1000
This strategy is known as snapshotting.
Whenever Redis needs to dump the dataset to disk, this is what happens:
Redis forks. We now have a child and a parent process.
The child starts to write the dataset to a temporary RDB file.
When the child is done writing the new RDB file, it replaces the old one.
This method allows Redis to benefit from copy-on-write semantics.
Snapshotting is not very durable. If your computer running Redis stops,
your power line fails, or you accidentally kill -9 your instance, the
latest data written to Redis will be lost. While this may not be a big
deal for some applications, there are use cases for full durability, and
in these cases Redis snapshotting alone is not a viable option.
The append-only file is an alternative, fully-durable strategy for Redis. It became available in version 1.1.
You can turn on the AOF in your configuration file:
appendonly yes
From now on, every time Redis receives a command that changes the
dataset (e.g. SET) it will append it to the AOF. When you restart
Redis it will re-play the AOF to rebuild the state.
Since Redis 7.0.0, Redis uses a multi part AOF mechanism. That is, the original single AOF file is split into base file (at most one) and incremental files (there may be more than one). The base file represents an initial (RDB or AOF format) snapshot of the data present when the AOF is rewritten. The incremental files contains incremental changes since the last base AOF file was created. All these files are put in a separate directory and are tracked by a manifest file.
The AOF gets bigger and bigger as write operations are performed. For example, if you are incrementing a counter 100 times, you'll end up with a single key in your dataset containing the final value, but 100 entries in your AOF. 99 of those entries are not needed to rebuild the current state.
The rewrite is completely safe. While Redis continues appending to the old file, a completely new one is produced with the minimal set of operations needed to create the current data set, and once this second file is ready Redis switches the two and starts appending to the new one.
So Redis supports an interesting feature: it is able to rebuild the AOF
in the background without interrupting service to clients. Whenever
you issue a BGREWRITEAOF, Redis will write the shortest sequence of
commands needed to rebuild the current dataset in memory. If you're
using the AOF with Redis 2.2 you'll need to run BGREWRITEAOF from time to
time. Since Redis 2.4 is able to trigger log rewriting automatically (see the
example configuration file for more information).
Since Redis 7.0.0, when an AOF rewrite is scheduled, the Redis parent process opens a new incremental AOF file to continue writing. The child process executes the rewrite logic and generates a new base AOF. Redis will use a temporary manifest file to track the newly generated base file and incremental file. When they are ready, Redis will perform an atomic replacement operation to make this temporary manifest file take effect. In order to avoid the problem of creating many incremental files in case of repeated failures and retries of an AOF rewrite, Redis introduces an AOF rewrite limiting mechanism to ensure that failed AOF rewrites are retried at a slower and slower rate.
You can configure how many times Redis will
fsync data on disk. There are
three options:
appendfsync always: fsync every time new commands are appended to the AOF. Very very slow, very safe. Note that the commands are appended to the AOF after a batch of commands from multiple clients or a pipeline are executed, so it means a single write and a single fsync (before sending the replies).appendfsync everysec: fsync every second. Fast enough (since version 2.4 likely to be as fast as snapshotting), and you may lose 1 second of data if there is a disaster.appendfsync no: Never fsync, just put your data in the hands of the Operating System. The faster and less safe method. Normally Linux will flush data every 30 seconds with this configuration, but it's up to the kernel's exact tuning.The suggested (and default) policy is to fsync every second. It is
both fast and relatively safe. The always policy is very slow in
practice, but it supports group commit, so if there are multiple parallel
writes Redis will try to perform a single fsync operation.
It is possible the server crashed while writing the AOF file, or the volume where the AOF file is stored was full at the time of writing. When this happens the AOF still contains consistent data representing a given point-in-time version of the dataset (that may be old up to one second with the default AOF fsync policy), but the last command in the AOF could be truncated. The latest major versions of Redis will be able to load the AOF anyway, just discarding the last non well formed command in the file. In this case the server will emit a log like the following:
* Reading RDB preamble from AOF file...
* Reading the remaining AOF tail...
# !!! Warning: short read while loading the AOF file !!!
# !!! Truncating the AOF at offset 439 !!!
# AOF loaded anyway because aof-load-truncated is enabled
You can change the default configuration to force Redis to stop in such cases if you want, but the default configuration is to continue regardless of the fact the last command in the file is not well-formed, in order to guarantee availability after a restart.
Older versions of Redis may not recover, and may require the following steps:
Make a backup copy of your AOF file.
Fix the original file using the redis-check-aof tool that ships with Redis:
$ redis-check-aof --fix <filename>
Optionally use diff -u to check what is the difference between two files.
Restart the server with the fixed file.
If the AOF file is not just truncated, but corrupted with invalid byte sequences in the middle, things are more complex. Redis will complain at startup and will abort:
* Reading the remaining AOF tail...
# Bad file format reading the append only file: make a backup of your AOF file, then use ./redis-check-aof --fix <filename>
The best thing to do is to run the redis-check-aof utility, initially without
the --fix option, then understand the problem, jump to the given
offset in the file, and see if it is possible to manually repair the file:
The AOF uses the same format of the Redis protocol and is quite simple to fix
manually. Otherwise it is possible to let the utility fix the file for us, but
in that case all the AOF portion from the invalid part to the end of the
file may be discarded, leading to a massive amount of data loss if the
corruption happened to be in the initial part of the file.
Log rewriting uses the same copy-on-write trick already in use for snapshotting. This is how it works:
Redis >= 7.0
Redis forks, so now we have a child and a parent process.
The child starts writing the new base AOF in a temporary file.
The parent opens a new increments AOF file to continue writing updates. If the rewriting fails, the old base and increment files (if there are any) plus this newly opened increment file represent the complete updated dataset, so we are safe.
When the child is done rewriting the base file, the parent gets a signal, and uses the newly opened increment file and child generated base file to build a temp manifest, and persist it.
Profit! Now Redis does an atomic exchange of the manifest files so that the result of this AOF rewrite takes effect. Redis also cleans up the old base file and any unused increment files.
Redis < 7.0
Redis forks, so now we have a child and a parent process.
The child starts writing the new AOF in a temporary file.
The parent accumulates all the new changes in an in-memory buffer (but at the same time it writes the new changes in the old append-only file, so if the rewriting fails, we are safe).
When the child is done rewriting the file, the parent gets a signal, and appends the in-memory buffer at the end of the file generated by the child.
Now Redis atomically renames the new file into the old one, and starts appending new data into the new file.
If you want to enable AOF in a server that is currently using RDB snapshots, you need to convert the data by enabling AOF via CONFIG command on the live server first.
IMPORTANT: not following this procedure (e.g. just changing the config and restarting the server) can result in data loss!
Redis >= 2.2
Preparations:
Switch to AOF on live database:
redis-cli config set appendonly yesredis-cli config set save ""redis.conf (potentially through CONFIG REWRITE) and ensure that it matches the configuration above.
If you forget this step, when you restart the server, the configuration changes will be lost and the server will start again with the old configuration, resulting in a loss of your data.Next time you restart the server:
INFO persistence, waiting for aof_rewrite_in_progress and aof_rewrite_scheduled to be 0, and validating that aof_last_bgrewrite_status is ok.Redis 2.0
redis-cli BGREWRITEAOF. This will create the append only file.Redis >= 2.4 makes sure to avoid triggering an AOF rewrite when an RDB
snapshotting operation is already in progress, or allowing a BGSAVE while the
AOF rewrite is in progress. This prevents two Redis background processes
from doing heavy disk I/O at the same time.
When snapshotting is in progress and the user explicitly requests a log
rewrite operation using BGREWRITEAOF the server will reply with an OK
status code telling the user the operation is scheduled, and the rewrite
will start once the snapshotting is completed.
In the case both AOF and RDB persistence are enabled and Redis restarts the AOF file will be used to reconstruct the original dataset since it is guaranteed to be the most complete.
Before starting this section, make sure to read the following sentence: Make Sure to Backup Your Database. Disks break, instances in the cloud disappear, and so forth: no backups means huge risk of data disappearing into /dev/null.
Redis is very data backup friendly since you can copy RDB files while the database is running: the RDB is never modified once produced, and while it gets produced it uses a temporary name and is renamed into its final destination atomically using rename(2) only when the new snapshot is complete.
This means that copying the RDB file is completely safe while the server is running. This is what we suggest:
find command to make sure too old snapshots are deleted: for instance you can take hourly snapshots for the latest 48 hours, and daily snapshots for one or two months. Make sure to name the snapshots with date and time information.If you run a Redis instance with only AOF persistence enabled, you can still perform backups.
Since Redis 7.0.0, AOF files are split into multiple files which reside in a single directory determined by the appenddirname configuration.
During normal operation all you need to do is copy/tar the files in this directory to achieve a backup. However, if this is done during a rewrite, you might end up with an invalid backup.
To work around this you must disable AOF rewrites during the backup:
CONFIG SET auto-aof-rewrite-percentage 0BGREWRITEAOF) during this time.INFO persistenceaof_rewrite_in_progress is 0. If it's 1, then you'll need to wait for the rewrite to complete.appenddirname directory.CONFIG SET auto-aof-rewrite-percentage <prev-value>Note: If you want to minimize the time AOF rewrites are disabled you may create hard links to the files in appenddirname (in step 3 above) and then re-enable rewrites (step 4) after the hard links are created.
Now you can copy/tar the hardlinks and delete them when done. This works because Redis guarantees that it
only appends to files in this directory, or completely replaces them if necessary, so the content should be
consistent at any given point in time.
Note: If you want to handle the case of the server being restarted during the backup and make sure no rewrite will automatically start after the restart you can change step 1 above to also persist the updated configuration via CONFIG REWRITE.
Just make sure to re-enable automatic rewrites when done (step 4) and persist it with another CONFIG REWRITE.
Prior to version 7.0.0 backing up the AOF file can be done simply by copying the aof file (like backing up the RDB snapshot). The file may lack the final part but Redis will still be able to load it (see the previous sections about truncated AOF files).
Disaster recovery in the context of Redis is basically the same story as backups, plus the ability to transfer those backups in many different external data centers. This way data is secured even in the case of some catastrophic event affecting the main data center where Redis is running and producing its snapshots.
We'll review the most interesting disaster recovery techniques that don't have too high costs.
gpg -c (in symmetric encryption mode). Make sure to store your password in many different safe places (for instance give a copy to the most important people of your organization). It is recommended to use multiple storage services for improved data safety.authorized_keys file of your small VPS. You are ready to transfer backups in an automated fashion. Get at least two VPS in two different providers
for best results.It is important to understand that this system can easily fail if not implemented in the right way. At least, make absolutely sure that after the transfer is completed you are able to verify the file size (that should match the one of the file you copied) and possibly the SHA1 digest, if you are using a VPS.
You also need some kind of independent alert system if the transfer of fresh backups is not working for some reason.