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Cache Disk

Important! This page is a basic introduction to the Unraid Cache drive, but was written for v4 and v5. There is no mention of Dockers, VM's, or Cache Pools. For v6 users, this page serves as an introduction, but you should also check these resources:

The cache disk feature of Unraid greatly increases the perceived performance of the system when writing data to a parity-protected array. It is particularly useful for users who write data to the array on a regular basis, and can also be used as a scratch disc for certain applications. The feature was introduced in version 4.3.

Overview​

A cache disk is a hard drive that is not part of the normal parity-protected array. When a cache disk exists in the system, it is visible as a disk share named 'cache' (provided disk shares are being exported). You may read data from or write data to the cache share just as you would any other disk share. As the cache disk is outside of the parity-protected array, writes will be much faster, but of course if this disk fails, all its data may be lost.

The real power of a cache disk is realized when User Shares are enabled. The cache disk may be automatically 'included' in every user share. Hence any object -- a file or directory - created on a user share is created on the cache disk (provided that enough space is available). Therefore, when you browse the files on a user share, the listings will transparently include objects on the cache disk as well as those on the other data disks.

In order to prevent the cache disk from filling up, a utility called the 'mover' will move objects from the cache disk to the array proper. You can set a schedule that defines when the mover will 'wake up'. The default schedule is to wake up at 3:40AM every day.

As there is a lag between the time objects are created on the cache disk, and when they are moved to the array, it may be desirable to disable the cache disk for certain shares. Each user share therefore has the option to disable the use of the cache disk for that share.

Adding a cache disk to the array​

In order to create a cache disk using the GUI:

  1. Stop the array
  2. Click on "Disk" at the top.
  3. Look at the very bottom of the page in the Disk Devices section for a disk 'slot' labeled Cache (the very last slot).
  4. Select any disk not in the array into that slot.
  5. Start the array

Cache settings​

This section is present on the Share Settings page when user shares are enabled, and a cache disk is present and formatted.

Min. free space specifies the minimum amount of free space that must exist on the Cache disk in order for the user share file system to create objects on the Cache disk. If there is less than this amount of free space then the object will be created on the array instead.

: Main article: Storage Management

Speed​

Perceived write speed increases​

The emphasis here is on 'perceived'. The real, behind-the-scenes write speed of your Unraid server is unchanged by the addition of a cache drive. A cache drive simply grants you the fastest transfer that your hardware will allow by deferring the inevitable parity calculation until a later time (3:40 am server time, by default).

Actual write speed increases​

How much of a write speed improvement can you expect to see from using a cache drive? That depends on your hardware. Slower hardware will of course result in slower transfer speeds. Assuming that you have Gigabit LAN (GigE) capable network controllers on both sides of the transfer (the client PC and the Unraid server), a Gigabit LAN router or switch, appropriate network cables (Cat5e or Cat6), and modern SATA I or better hard drives, then you should see write speeds in the following ranges:

These numbers represent average transfer speeds when writing data to Unraid:

Without a cache drive: Unraid 4.5.3 - average 20-30 MB/s, peak reported 40 MB/s*

With a cache drive: Unraid 4.5.3 - average 50-60 MB/s, peak reported 101 MB/s*

So generally speaking, a server with a cache drive has write speeds 2-3x faster than the same server without a cache drive.

  • Note: These figures (especially the averages) are based upon personal observations by Rajahal and upon reports from other reputable sources in these forums.

The Mover​

The Mover is a utility that runs periodically to move files off the cache disk and onto parity-protected data disk(s). If Mover logging is enabled, the mover will record all its activity to the system log.

By default the mover is scheduled to run every day at 3:40AM. This may be changed by defining your own Mover schedule string in crontab format.

If the mover finds no files to move, and the disks are spun-down, the disks will not spin up. Conversely, file(s) will only be moved if they are not open for reading/writing -- they will move the next night, when they are no longer open.

: Main article: Storage Management

  • The mover will not move any top-level directories which begin with a '.' character. Such directories will not exist in normal use, but an advanced user may use this knowledge to create directories which won't get moved.

  • The mover will not move any files that exist in the root of the cache disk. Such files will not exist in normal use, but an advanced user may use this knowledge to create files which won't get moved (for example, a swap file).

  • The mover is just a script called '/usr/local/sbin/mover' which invokes 'find' to traverse the cache disk and move files to the array using the 'mv' command. Advanced users may edit this script to fine-tune the mover. For example, it's possible to set conditions such as 'move only files older than N days', or 'only move files greater than N bytes in size', etc. Refer to the script itself and the 'man' page of the 'find' command.

Other uses for a cache disk​

Warm spare​

The purpose of a 'warm spare' is to minimize the amount of time that your array is without parity protection after a drive failure. A warm spare is a hard drive that you have installed in your server to prepare for the eventuality of one of your other hard drives failing. It requires that the warm spare drive be as large or larger than your parity drive and your largest data drive

When a drive does eventually fail, you simply stop the array (via the Main page), unassign the dead drive (via the Devices page), assign the warm spare (also on the Devices page), and then start the array (back on the Main page). At this point, your data from the dead drive will be automatically rebuilt onto your warm spare - this will take many hours, but your array and all your data is still available for use during this time (though performance will likely be degraded).

In the classic application, a Warm Spare sits in your server pre-installed (therefore taking up a SATA/PATA slot and using a small amount of power), but constantly spun down and unused (as it is not assigned to a disk slot in Unraid).

However, why not use a warm spare as a cache drive while you wait around for another drive to fail? This application will of course add a bit of wear and tear to your warm spare, but nothing outside the scope of ordinary use.

Running other software on top of Unraid​

As a cache drive is outside of the parity-protected array, it can be used for alternate software and Unraid add-ons that need to read and write data often.

Scratch space​

The cache disk can be used as scratch space for torrent client(s), news clients or web servers. The advantage here is that, for most of the day, only the one cache disk in the array needs to be spun up. Otherwise, at least two drives - parity and one data -- will be spun up.

Disadvantages of a cache disk​

'Wasted' HDD and HDD slot​

One of your hard drives and SATA/PATA slots is dedicated to your cache drive, instead of long-term data storage

Short-term risk of data loss​

If your cache drive dies, it will take all of the data currently residing on it to the grave. While it is possible to run ReiserFS data recovery software on your dead cache drive, there is still a good possibility of permanent data loss.

By default, the mover script will run once a day at 3:40 am. If you are concerned about data loss, you can edit the script so that it runs more frequently and at different times.

Hardware​

The two key factors to consider to when choosing a hard drive to be your cache disk are:

  • Your intended purpose.
  • Amount of data

Purpose​

Increased perceived write speed: You will want a drive that is as fast as possible. For the fastest possible speed, you'll want an SSD (which has the added benefit of being low power) or a 10,000 rpm drive. If you are on a tighter budget, a 7200 rpm drive such as a WD Black will do fine. Unless you write a lot of very small files, the size of the hard drive's cache (8, 16, 32, 64 mb etc.) won't matter much. You can also eek a bit more performance out of a slower drive by short stroking it (meaning confine it to only the outside sectors, which are higher density and can therefore be written to and read from faster), but this is of course an advanced maneuver.

As a Warm Spare: you will want a drive that is the same size or larger than your parity disk. This drive can be any speed you choose, just remember that in the event of a parity rebuild (if your parity disk dies) or a rebuild from parity (if a data disk dies), the process will proceed at the speed of your slowest disk.

Amount of data​

The final consideration in choosing a cache drive is to think about the amount of data you expect to pass through it. If you write ~10 GBs per day, then any drive 10 GB or larger will do (a 30 GB SSD may be a good fit in this case). If you write 100 GB in one day every few weeks, then you will want a cache drive that is larger than 100 GB. If you attempt a data transfer that is larger than the size of your cache drive, the transfer will fail.