ReadyNAS OS 6 devices use a file system with an internal structure quite different from traditional file systems. While this modern approach has advantages like high performance disc operations, unlimited snapshots, and advanced data corruption protection, it also requires the introduction of new maintenance concepts. ReadyNAS users do not need to have intrinsic knowledge of these as they can easily set up their devices to automatically take care of any maintenance action required.Nevertheless, this article shall give a short overview of the tasks available and explain the influence these maintenance tasks have on free space available.
Scrubbing
What is scrubbing?
One of the ReadyNAS features to protect user data is bitrot protection. In short, whenever data is stored on the ReadyNAS device an additional piece of data is written. This extra data is unique to the original data stored and can be used to discover inadvertent alteration of the original data. Whenever a file is read a background check is performed to verify if the data is still consistent.
Scrubbing checks all stored data for consistency, and therefore will also discover potential errors in files that have not been accessed in a long time. If multiple hard discs are used in a RAID Array Level 1 or greater then scrubbing will also correct any errors found automatically. Scrubbing does not delete any data on the device.
Should I use scrubbing regularly?
Scrubbing is closely connected to the bitrot protection feature. If bitrot protection is turned off, regular scrubbing is not required. Scrubbing should not be executed excessively as it puts considerable stress on the hard discs. The impact on available free space is limited.
How do I scrub a volume?
Scrubbing can be run manually or configured to run automatically via the local admin page.
Defragmentation
What is defragmentation?
Storage space is divided into blocks that can hold a certain amount of data. New data is stored in blocks that are free or blocks still occupied by data but marked as free. As the distribution of available blocks on the storage medium depends on past disc operations there might not be a sufficiently large number of contiguous blocks to hold the complete file. Thus, the data of a single file is spread across different locations on the storage medium, fragmenting the file. Defragmentation rearranges all data in order to have each file stored in a line of contiguous blocks.
How is fragmentation different on ReadyNAS devices compared to traditional storage devices?
Basically, fragmentation occurs on ReadyNAS devices as it would on most mass storage devices. However, there is one difference with ReadyNAS devices. When the bitrot protection functionality is enabled the ReadyNAS device moves even further away from traditional storage devices in the way it handles files. One of the effects is that new files are stored only on genuinely free blocks in contrast to the options mentioned above. This leads to a slightly higher rate of fragmentation
Should I use defragmentation regularly?
Fragmentation produces a considerable amount of data overhead, so defragmenting a volume can lead to an increase in free space available. Read performance will most likely also be increased when using magnetic hard disc drives.
However, in some cases defragmentation can also lead to reduced free space available. This again is connected to the new methods implemented in the ReadyNAS file system. More precisely, defragmentation can lead to losing the advantage of these methods as you might be losing the advantage of zero size snapshots for existing snapshots. Basically, snapshots are disconnected from the data they are pointing to and are therefore manifested on the storage device taking up additional disc space. Finally note that defragmentation puts stress on the hard discs.
Taking all into account, defragmentation is advisable at sparse regular intervals.
How do I defragment a volume?
Defragmentation can be run manually or configured to run automatically via the local admin page. Defragmentation is not necessary when using SSD drives due to constant seek time independent of storage location.
Balancing
What is balancing?
The ReadyNAS file system not only stores the user data, it also stores additional information related to that data, called metadata. Either data is stored in a number of fixed size storage areas called chunks which are reserved by the file system when new data is stored. In normal operation a chunk reserved for user data cannot be converted to hold metadata, even when the chunk becomes empty again. Therefore, a progression from a high number of large files to a higher number of small files stored on the ReadyNAS can lead to a shortage of available metadata chunks. This can lead to a situation where the ReadyNAS device will report that is has run out of disc space even when enough space seems to be available. Running the balance function will combine half-filled chunks and free empty, previously assigned chunks for reuse with either user data or metadata.
Should I use balancing regularly?
Since balancing requires some storage space to move files it should not be postponed until the available storage space is almost consumed completely. The balancing function will be most effective after a large number of files have been deleted or if there has been a transition from generally large files to smaller files. Especially the latter is hard to monitor, therefore balancing should be run at regular intervals.
How do I balance a volume?
Balancing can be configured to run automatically via the local admin page.
Snapshots
The topic of snapshots is somewhat misplaced in this list. However, snapshots have an indirect influence on available free space as well.
There may be a case where after deletion of a file none of the methods mentioned above help to free up space previously occupied by that file. The reason for that is most likely that one or several snapshots made of that file still exist.
While there is an automatic function build in the ReadyNAS OS to limit the space taken up by snapshots over time by removing some of the older snapshots, this function will not eventually delete all of the snapshots as this would undermine the reason to use snapshots.
This requires intervention of the user by manually deleting all snapshots of that file.
As a final comment concerning free space it has to be noted that there may be a small latency between deletion of a file or snapshot and the indication of additional free space.