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launch arguments

You can launch the program with several different arguments to alter core behaviour. If you are not familiar with this, you are essentially putting additional text after the launch command that runs the program. You can run this straight from a terminal console (usually good to test with), or you can bundle it into an easy shortcut that you only have to double-click. An example of a launch command with arguments:

C:\Hydrus Network\hydrus_client.exe -d="E:\hydrus db" --no_db_temp_files

You can also add --help to your program path, like this:

  • --help
  • hydrus_server.exe --help
  • ./hydrus_server --help

Which gives you a full listing of all below arguments, however this will not work with the built hydrus_client executables, which are bundled as a non-console programs and will not give you text output to any console they are launched from. As hydrus_client.exe is the most commonly run version of the program, here is the list, with some more help about each command:

-d DB_DIR, --db_dir DB_DIR

Lets you customise where hydrus should use for its base database directory. This is install_dir/db by default, but many advanced deployments will move this around, as described here. When an argument takes a complicated value like a path that could itself include whitespace, you should wrap it in quote marks, like this:

-d="E:\my hydrus\hydrus db"

--temp_dir TEMP_DIR

This tells all aspects of the client, including the SQLite database, to use a different path for temp operations. This would be by default your system temp path, such as:


But you can also check it in help->about. A handful of database operations (PTR tag processing, vacuums) require a lot of free space, so if your system drive is very full, or you have unusual ramdisk-based temp storage limits, you may want to relocate to another location or drive.


Change the journal mode of the SQLite database. The default is WAL, which works great for almost all SSD drives, but if you have a very old or slow drive, or if you encounter 'disk I/O error' errors on Windows with an NVMe drive, try TRUNCATE. Full docs are here.


  • WAL - Clever write flushing that takes advantage of new drive synchronisation tools to maintain integrity and reduce total writes.
  • TRUNCATE - Compatibility mode. Use this if your drive cannot launch WAL.
  • PERSIST - This is newly added to hydrus. The ideal is that if you have a high latency HDD drive and want sync with the PTR, this will work more efficiently than WAL journals, which will be regularly wiped and recreated and be fraggy. Unfortunately, with hydrus's multiple database file system, SQLite ultimately treats this as DELETE, which in our situation is basically the same as TRUNCATE, so does not increase performance. Hopefully this will change in future.
  • MEMORY - Danger mode. Extremely fast, but you had better guarantee a lot of free ram and no unclean exits.

--db_transaction_commit_period DB_TRANSACTION_COMMIT_PERIOD

Change the regular duration at which any database changes are committed to disk. By default this is 30 (seconds) for the client, and 120 for the server. Minimum value is 10. Typically, if hydrus crashes, it may 'forget' what happened up to this duration on the next boot. Increasing the duration will result in fewer overall 'commit' writes during very heavy work that makes several changes to the same database pages (read up on WAL mode for more details here), but it will increase commit time and memory/storage needs. Note that changes can only be committed after a job is complete, so if a single job takes longer than this period, changes will not be saved until it is done.

--db_cache_size DB_CACHE_SIZE

Change the size of the cache SQLite will use for each db file, in MB. By default this is 256, for 256MB, which for the four main client db files could mean an absolute 1GB peak use if you run a very heavy client and perform a long period of PTR sync. This does not matter so much (nor should it be fully used) if you have a smaller client.

--db_synchronous_override {0,1,2,3}

Change the rules governing how SQLite writes committed changes to your disk. The hydrus default is 1 with WAL, 2 otherwise.

A user has written a full guide on this value here! SQLite docs here.


When SQLite performs very large queries, it may spool temporary table results to disk. These go in your temp directory. If your temp dir is slow but you have a ton of memory, set this to never spool to disk, as here.


Prints additional debug information to the log during the bootup phase of the application.


This starts the program with 'Profile Mode' turned on, which captures the performance of boot functions. This is also a way to get Profile Mode on the server, although support there is very limited.


Windows only, client only: This starts the program with Qt's 'darkmode' detection enabled, as here, set to 1 mode. It will override any existing qt.conf, so it is only for experimentation. We are going to experiment more with the 2 mode, but that locks the style to windows, and can't handle switches between light and dark mode.

server arguments

The server supports the same arguments. It also takes an optional positional argument of 'start' (start the server, the default), 'stop' (stop any existing server), or 'restart' (do a stop, then a start), which should go before any of the above arguments.