When not in synchronous 0 mode, the database engine syncs at regular intervals to make sure data has been written. A program can also request that a file it has just written to be flushed or synced, and it will wait until that is done before continuing. One purpose of a normal shutdown of the operating system is to make sure all disk caches have been flushed and synced. The caveat is that until you have ' synced' the disk cache, the changes to files are not actually in permanent storage. This is called write cache and it is really important to use it or your system's performance would be terrible. In the normal operation of your system, files are constantly being partially transfered to disk, even if the OS pretends they have been fully written to disk. Setting the synchronous to 0 lets the database engine defer writing to disk as long as physically possible. Note: In historical versions of hydrus ( synchronous=2), performance was terrible because hydrus would agressively (it was arguably somewhat paranoid) write changes to disk.

Normal shutdown may also take abnormally long because the system is flushing these pending writes, but you must allow it to take its time as explained in the section below.

Because of the potential for a lot of outstanding writes when using synchronous=0, other I/O on your system will slow down as the pending writes are interleaved.

Restore the sqlite database files backup if the OS shutdown abnormally.

Exit hydrus normally and let the OS flush disk caches (either by letting the system run/'idle' for a while, running sync on *NIX systems, or normal shutdown), or.

After running with synchronization set to zero you must either:.

If you turn synchronization to 0 you are gambling, but it is a safe gamble if you have a backup and know exactly what you are doing.

SSDs are 10-100x as slow as memory, and HDDs are 1000-10000x as slow as memory.

It is always more expensive to access the disk than doing things in memory.

It is nearly always safe to use -db_synchronous_override=1 on any modern filesystem and this is the default.

This is a tutorial for advanced users who have read and understood this document and the risk/recovery procedure.

Tuning your database synchronization using the -db_synchronous_override=0 launch argument can make Hydrus significantly faster with some caveats. Understanding Database Synchronization Options ¶

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