The Cache Pool & the Mover, Made Simple

What the Cache Actually Is

The cache is simply a fast drive that sits in front of your main storage. When you save a file, it lands on the cache first, so the save finishes quickly. Later, the file moves to the array where there is lots of room.

Think of a busy kitchen. The counter by the stove is small but close at hand - you drop things there while you cook. The pantry is big but farther away. The cache is that counter. The array is the pantry. You work fast on the counter, then put things away in the pantry when you have a moment.

• Saving files feels quick, because the fast drive answers first.
• The main array drives can stay asleep more often, so the server is quieter.
• Apps that need speed can live on the cache full-time.

Why an SSD Cache Makes Saving Files Fast

The array is protected by a parity drive, and that protection has a cost: every write has to update parity too, which slows things down. A cache drive skips that step for now, so your save finishes right away.

An SSD - a solid-state drive with no moving parts - is also just plain faster than a spinning hard drive. Put those two things together and saving a big folder to the cache can feel many times quicker than saving it straight to the array.

• Copying files over the network finishes sooner.
• The server does not grind and whir on every little save.
• Apps that read and write often feel snappy instead of sluggish.

The "Pool" Idea

In newer Unraid, the cache is called a "pool." A pool is just one or more fast drives grouped together and given a name. You can even have more than one pool for different jobs.

Do not let the word trip you up. A pool with a single SSD is still just "your cache." A pool with two SSDs lets them mirror each other for safety. The word "pool" simply lets Unraid handle one drive or several under one tidy label.

• One cache pool - the classic setup, a fast landing spot for new files.
• A mirrored pool - two SSDs that keep the same data, so one can fail safely.
• A separate pool - some folks keep a second pool just for app settings.

The Per-Share Cache Settings, in Plain Words

Each share (a folder like Media or Documents) has a setting that decides how it uses the cache. In current Unraid these are named after where files go and how the Mover treats them. Here they are in everyday language.

If your Unraid uses newer wording (like a "primary" and "secondary" storage with a move direction), it is the same four ideas dressed differently. The point is: does this share use the cache, and which way does the Mover carry its files.

How the Mover Works

The Mover is a simple background helper. Its whole job is to carry files off the cache and onto the array so the cache stays free for new work. Think of it as someone who clears the counter and puts things in the pantry.

When the Mover runs, it looks at shares set to "Yes" and moves their cached files onto the array. Shares set to "Only" or "Prefer" are left where they are on purpose. You do not have to do anything - it runs on its own.

• It does not delete your files - it only relocates them.
• It does not touch "Only" shares like appdata.
• It does not run constantly - it runs on a schedule you can see.

The Mover's Schedule

By default the Mover runs once a day, often in the small hours of the morning. That timing is on purpose: it does its shuffling while you are asleep and not using the server.

You can change how often it runs - hourly, daily, weekly - in the scheduler settings. For most homes, the once-a-day default is exactly right. Running it too often can add wear and noise for little benefit.

• You fill the cache fast - a heavy download habit may want a more frequent move.
• You never fill it - a light user can leave it daily and forget about it.
• Noise matters at night - pick a time when a little disk activity will not bother you.

Why App Settings Usually Stay on the Cache

"Appdata" is the folder that holds the settings and small working files for your apps - things like your Plex library details or a container's configuration. Apps read and write to it constantly.

Because apps hit appdata so often, you want it on the fast cache, not the slower array. Keeping it on the cache also lets the big array drives sleep. That is why appdata is normally set to "Only" or "Prefer" - it lives on the cache and the Mover leaves it be.

Keeping Downloads on the Cache

If you download files - especially large ones that are still being assembled - it is kinder to your array to do that work on the cache. Downloads often write in bursts and pieces, which spinning drives dislike.

A common setup is a "downloads" share set to "Yes" so files finish downloading on the fast cache, and only the completed file gets moved to the array later. That keeps the array drives calm and your downloads speedy.

• In-progress files thrash the disk; the SSD shrugs that off.
• The big array drives can stay asleep during a download.
• The finished file moves to the array clean and whole.

Using Two Cache Drives for Protection

A single cache drive has no safety net - if it dies, whatever lived only on it is gone. The fix is a second cache drive that mirrors the first, so the two hold the same data.

With two drives in a mirrored pool, either one can fail and your data is still safe on the other. You swap in a replacement and the pool heals itself. This is the standard way to protect appdata that lives on the cache.

• Yes, if you keep important app settings or files on the cache full-time.
• Maybe not, if your cache only ever holds files on their way to the array.
• Either way, a backup still matters - a mirror is not a backup.

The Danger of a Cache Filling Up

The most common cache trouble is simple: it fills up. When there is no free space left on the cache, new files have nowhere fast to land, and things start to misbehave.

A full cache can cause copies to fail, downloads to stall, and in some cases apps to act up because they cannot write. It is not usually dangerous to your data, but it is annoying and it is the thing to watch for.

• You copied more at once than the cache can hold before the Mover runs.
• A share set to "Only" or "Prefer" is quietly growing and eating the space.
• The Mover has not run yet, or is set to run too seldom.

Watching How Full the Cache Is

You do not have to guess. The Unraid dashboard shows each drive and pool with a little used-space bar. A glance tells you how much room the cache has left.

Unraid can also warn you automatically. In the notification settings you can set a disk-usage alert so you get a heads-up by the time the cache is, say, 70 or 90 percent full - long before it becomes a problem.

• Open the Unraid web page and look at the Main tab.
• Find your cache pool in the list of devices.
• Check the used and free space, or the little colored bar.
• If it is getting full, run the Mover or investigate what is filling it.

Moving Appdata Around Safely

Sometimes you need to move appdata - maybe onto a new cache pool, or off a cache that is filling up. It is straightforward if you do it in the right order, with the apps turned off.

The one rule that saves grief: never move appdata while the apps are running. A container writing to a folder that is being moved underneath it is how settings get corrupted.

• Back up your appdata folder first, before you touch anything.
• Stop the apps (the Docker containers) so nothing is writing.
• Change the share's cache setting to send files the direction you want.
• Run the Mover, and let it finish completely before moving on.
• Set the share back to "Only" or "Prefer" for its new home, then start the apps.

When NOT to Use a Cache

A cache is helpful, but it is not right for every share. Sometimes sending files straight to the array is the smarter choice. Knowing when saves you from odd surprises.

For those cases, set the share to "No" so files go directly to the array. Yes, saving is a bit slower, but the file is protected right away and you never risk overflowing the cache with one giant copy.

• You are writing something huge that is bigger than the cache itself.
• You want the file protected by parity the instant it lands, not hours later.
• The share holds archive files you rarely touch - speed does not matter there.

SSD Wear, Briefly

Solid-state drives can only be written to so many times before they wear out. That sounds scary, but for a home server it almost never matters. Modern SSDs last many years of normal use.

Because the cache handles a lot of writing, it does see more wear than a drive that just sits there. Even so, a decent SSD is rated for far more writing than a typical home will ever do. It is worth knowing about, not worth worrying about.

• Buy a reputable SSD rather than the cheapest unknown brand.
• Do not stuff the cache to 100 percent full and leave it there.
• Keep a backup, as you would for any drive - wear or not.

TRIM, Briefly

TRIM is a little housekeeping task that helps an SSD stay fast over time. When you delete files, TRIM tells the SSD which spots are now free, so it can keep writing efficiently.

You do not need to understand the technical details. Unraid can run TRIM on a schedule for you, usually through a small plugin or a built-in setting. Turn it on and forget it - that is the whole job.

• Make sure TRIM is scheduled for your SSD cache (a plugin or setting handles it).
• Let it run automatically - once a day or once a week is plenty.
• Note that some drive setups handle TRIM differently; when unsure, ask.

Troubleshooting the Mover

If files are not leaving the cache when you expect, the Mover is rarely broken - usually a setting is quietly telling it to leave those files alone. Here is the calm order to check.

A very common one: a file will not move because an app still has it open. Stop the app, run the Mover again, and it usually goes through cleanly.

• Check the share's cache setting: "Only" and "Prefer" keep files ON the cache by design.
• Confirm the Mover schedule - it may simply not have run yet.
• Try the "Move Now" button to run it on demand and watch what happens.
• Make sure the array is started; the Mover needs it running to move files onto it.
• Look at the logs or notifications for any error about space or a busy file.

A Recommended Simple Setup

If you just want a sensible starting point that works for most homes, here it is. You can refine later, but this will serve you well from day one.

Add a scheduled appdata backup and a cache-usage alert, and you have a setup that is fast, quiet, and safe - without any fussing.

• One SSD cache pool - or two mirrored SSDs if you can, for safety.
• Appdata share set to keep files on the cache ("Only" or "Prefer").
• Everyday file shares set to "Yes" so they cache, then move to the array.
• Downloads share set to "Yes" so downloads finish on the fast drive.
• Mover left on its once-a-day overnight schedule.

Common Myths About the Cache

There is a lot of confusing chatter online. Let us clear up the myths that trip people up most, so you can ignore the noise.

When in doubt, come back to the simple picture: the cache is a fast counter in front of your big pantry, and the Mover puts things away. Everything else is detail.

What's in the 27-page PDF

• What the cache is, in everyday words
• Why a fast SSD cache makes saving files feel instant
• How the Mover works and when it runs
• The per-share cache settings, explained without the jargon
• Why app settings and downloads like to live on the cache
• Keeping the cache from filling up, and a simple recommended setup

Why I wrote this

I write these as printable handouts because a guide you can keep in a drawer beats a web page you have to find again. No jargon, no judgment, no rushing. If you'd rather have someone sit beside you and go through it together, that's a flat $99 first visit with the 30-day follow-up included — serving Portage County from Atwater, Ohio. — Bill