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Data migration


Upgrade to JuiceFS 4.9 to use the sync subcommand.

juicefs sync is a powerful data migration tool, which can copy data across all supported storages including object storage, JuiceFS itself, and local file systems, you can freely copy data between any of these systems. In addition, it supports remote directories through SSH, HDFS, WebDAV, etc. while providing advanced features such as incremental synchronization, and pattern matching (like rsync), and distributed syncing.

For data migrations that involve JuiceFS, it's recommended use the jfs:// protocol, rather than mount JuiceFS and access its local directory, which bypasses the FUSE mount point and access JuiceFS directly. This process still requires the client configuration file, you should prepare in advance using juicefs auth. Under large scale scenarios, bypassing FUSE can save precious resources and increase performance.

juicefs sync works like this:

juicefs sync [command options] SRC DST

# Sync object from OSS to S3
juicefs sync oss:// s3://

# Sync objects from S3 to JuiceFS
juicefs sync s3:// jfs://VOL_NAME/

# SRC: a1/b1,a2/b2,aaa/b1 DST: empty sync result: aaa/b1
juicefs sync --exclude='a?/b*' s3:// jfs://VOL_NAME/

# SRC: a1/b1,a2/b2,aaa/b1 DST: empty sync result: a1/b1,aaa/b1
juicefs sync --include='a1/b1' --exclude='a[1-9]/b*' s3:// jfs://VOL_NAME/

# SRC: a1/b1,a2/b2,aaa/b1,b1,b2 DST: empty sync result: a1/b1,b2
juicefs sync --include='a1/b1' --exclude='a*' --include='b2' --exclude='b?' s3:// jfs://VOL_NAME/


Simply put, when using sync to transfer big files, progress bar might move slowly or get stuck. If this happens, you can observe the progress using other methods.

sync assumes it's mainly used to copy a large amount of files, its progress bar is designed for this scenario: progress only updates when a file has been transferred. In a large file scenario, every file is transferred slowly, hence the slow or even static progress bar. This is worse for destinations without multipart upload support (e.g. file, sftp, jfs, gluster schemes), where every file is transferred single-threaded.

If progress bar is not moving, use below methods to observe and troubleshoot:

  • If either end is a JuiceFS mount point, you can use juicefs stats to quickly check current I/O status.
  • If destination is a local disk, look for temporary files that end with, these are the temp files created by sync, they will be renamed upon transfer complete. Look for size changes in temp files to verify the current I/O status.
  • If both end are object storage services, use tools like nethogs to check network IO.

Trailing slash

SRC and DST ending with a trailing / are treated as directories, e.g. movie/. Those don't without a trailing / are treated as prefixes, and will be used for pattern matching. Assuming we have test and text directories in the current directory, the following command can synchronize them into the destination ~/mnt/:

juicefs sync ./te ~/mnt/te

In this way, the subcommand sync takes te as a prefix to find all the matching directories, i.e. test and text. ~/mnt/te is also a prefix, and all directories and files synchronized to this destination will be renamed by replacing the original prefix te with the new prefix te. The changes in the names of directories and files before and after synchronization cannot be seen in the above example. However, if we take another prefix, for example, ab:

juicefs sync ./te ~/mnt/ab

The test directory synchronized to the destination directory will be renamed as abst, and text will be abxt.

Incremental and full synchronization

By default, juicefs sync performs incremental synchronization, existing files are overwritten when their sizes don't match. On top of this, you can also use --update to copy files when source files' mtime are newer. For scenarios with higher demand for data integrity, use --check-new or --check-all to perform byte stream comparison between SRC and DST.

For full synchronization (copy everything no matter already exists or not), use --force-update.

Pattern matching

Similar to rsync, you can use --exclude, --include patterns to build filters:

  • Patterns ending with / only matches directories; otherwise, they match files, links or devices.
  • Patterns containing *, ? or [ match as wildcards, otherwise, they match as regular strings;
  • * matches any non-empty path components (it stops at /).
  • ? matches any single character except /;
  • [ matches a set of characters, for example [a-z] or [[:alpha:]];
  • Backslashes can be used to escape characters in wildcard patterns, while they match literally when no wildcards are present.
  • It is always matched recursively using patterns as prefixes.
  • The earlier options have higher priorities than the latter ones. Thus, the --include options should come before --exclude. Otherwise, all the --include options such as --include 'pic/' --include '4.png' which appear later than --exclude '*' will be ignored.

To sync everything, but excludes hidden directories and files (name starting with . is regarded as hidden):

# Excluding hidden directories and files
juicefs sync --exclude '.*' /tmp/dir/ s3://

You can use this option several times with different parameters to exclude multiple patterns. For example, using the following command can exclude all hidden files, pic/ directory and 4.png:

juicefs sync --exclude '.*' --exclude 'pic/' --exclude '4.png' /tmp/dir/ s3://

Option --include can be used to include patterns you don't want to exclude. For example, only pic/ and 4.png are synchronized and all the others are excluded:

juicefs sync --include 'pic/' --include '4.png' --exclude '*' /tmp/dir/ s3://

Directory structure and file permissions

juicefs sync skips empty directories by default. To synchronize empty directories, you can use --dirs option.

In addition, when synchronizing between file systems such as local, SFTP and HDFS, option --perms can be used to synchronize file permissions.

You can use --links option to disable symbolic link resolving when synchronizing local directories. That is, synchronizing only the symbolic links themselves rather than the directories or files they are pointing to. The new symbolic links created by the synchronization refer to the same paths as the original symbolic links without any conversions, no matter whether their references are reachable before or after the synchronization. Also note that:

  • The mtime of a symbolic link will not be synchronized;
  • The --check-new and --perms options will be ignored when synchronizing symbolic links.

Concurrent data synchronization

juicefs sync by default starts 10 threads to run syncing jobs, you can set the --threads option to increase or decrease the number of threads as needed. But also note that due to various factors, blindly increasing --threads may not always work, and you should also consider:

  • SRC and DST storage systems may have already reached their bandwidth limits, if this is indeed the bottleneck, further increasing concurrency will not improve the situation;
  • Performing juicefs sync on a single host may be limited by host resources, e.g. CPU or network throttle, if this is the case, consider using distributed synchronization (introduced below);
  • If the synchronized data is mainly small files, and the list API of SRC storage system has excellent performance, then the default single-threaded list of juicefs sync may become a bottleneck. You can consider enabling concurrent list (introduced below).

Concurrent list

From the output of juicefs sync, pay attention to the Pending objects count, if this value stays zero, consumption is faster than production and you should increase --list-threads to enable concurrent list, and then use --list-depth to control list depth.

For example, if you're dealing with a object storage bucket used by JuiceFS, directory structure will be /<vol-name>/chunks/xxx/xxx/..., using --list-depth=2 will perform concurrent listing on /<vol-name>/chunks which usually renders the best performance.

Distributed synchronization

Synchronizing between two object storages is essentially pulling data from one and pushing it to the other. The efficiency of the synchronization will depend on the bandwidth between the client and the cloud.


When copying large scale data, node bandwidth can easily bottleneck the synchronization process. For this scenario, juicefs sync provides a multi-machine concurrent solution, as shown in the figure below.


Manager node executes sync command as the master, and defines multiple worker nodes by setting option --worker (manager node itself also serve as a worker node). JuiceFS will split the workload distribute to Workers for distributed synchronization. This increases the amount of data that can be processed per unit time, and the total bandwidth is also multiplied.

When using distributed syncing, you should configure SSH logins so that the manager can access all worker nodes without password, if SSH port isn't the default 22, you'll also have to include that in the manager's ~/.ssh/config. Manager will distribute the JuiceFS Client to all worker nodes, so they should all use the same architecture to avoid running into compatibility problems.

For example, to synchronize data between two object storage services:

juicefs sync --worker bob@,tom@ s3:// oss://

The synchronization workload between the two object storages is shared by the manager machine and the two Workers bob@ and tom@

Sync across regions using S3 Gateway

When transferring a large amount of small files across different regions via FUSE mount points, clients will inevitably talk to the metadata service in the opposite region via public internet (or dedicated network connection with limited bandwidth). In such cases, metadata latency can become the bottleneck of the data transfer:

sync via public metadata service

S3 Gateway comes to rescue in these circumstances: deploy a gateway in the source region, and since this gateway accesses metadata via private network, metadata latency is eliminated to a minimum, bringing the best performance for small file intensive scenarios.

sync via gateway

Read S3 Gateway to learn its deployment and use.