Minimum recommended space free reservations

Inspired by the following LT staffer personal recommendation:

Personally, I strive to keep disk usage below 90% because anything above that could degrade performance for writes on certain filesystems.

There are few follow on discussions but in general terms everyone seems to agree that:

• You should never fill up a drive to 100%
  
• You should ALWAYS reserve a certain amount of free space
  
• The amount of reserved free space depends on the filesystem type
  
• The amount of free space reserved depends on the use case (i.e. Predominately RO disks need less reservation that RW disks).
  

What is not agreed upon is:

• Should the reservation be a percentage, a GB amount or a combination of both.
  
• The differences each FS type makes (e.g. anecdotal evidence suggests XFS needs hardly any reservation whereas RFS needs comparitively lots).
  

This is an important topic as people will (and should) try to follow recommendations. More so, with the advent of 8TB + disks a strict adherence to the "10%" reservation recommendation amounts to 18.4TB of unused reserved space on a fully populated array (which is obviously a lot).

Google yields lots of other personal recommendations from a the tiny (few MB) to the insane (50%), but no hard facts.

The aim of this thread is to agree on a reservation amount recommendations for unRAID users of each supported filesystem type (if different). Ideally this should be a formal statement but failing hard facts a consensus of personal recommendations would best.

Thoughts?

I use a combination of percentage and amount.  With 4TB drives I like to keep at least 250GB free.  With 6TB I like to bump that up closer to 400GB free. They are very close to being the same percentage and the 6TB drives are the ones most likely to break my rule.

Disclaimer: I'm only at 51% of my 21TB array so I will be adding more space way before I have to consider this.

I set my warning level at 900GB (nearest percentage equivalent for each drive) and critical at 5% ("flat rate" for all drives).

I reckon (900GB x 4 drives) - (5% * 21TB) = 3.6TB - 1.05TB = 2.55TB. That would allow plenty of allowance while I wait for delivery of another HDD.

What we know -

* There are numerous reports of issues with drives that are ReiserFS formatted, if there has been considerable file management (adding and removing files in bunches), AND the drive is close to full.  There are a few reports of delays so long that they caused operations to fail, streams to be stopped with errors, network transfers stopped with errors about non responsive server, all because the Reiser file system took too long doing some sort of file system maintenance.  So for ReiserFS formatted drives expected to do considerable amounts of reading, writing, and deleting, it seems a good idea to recommend a fairly large free space percentage.  I've always liked a number between 90% and 95%, as I think you can often approach 95% before you get into trouble, but 90% is a safer and more conservative number.  If you plan to just fill the drive up, then use it that way with very few changes, then you should have no trouble formatting it as ReiserFS and filling it to near 100%.

* For XFS and BTRFS, we have had less than 2 years of unRAID experience, but so far have seen no issues at all with a full disk, no matter how used.  So unless something changes, we can't recommend less than "close to 100%", for any usage.

* However(!), as a general principle, if you are going to use a drive for heavy file reading and writing and deletion (load a bunch of files, remove a bunch, load a bunch more, etc etc), then you are not doing your file system any favors by filling it too close to 100%.  No matter what the file system is, you should not be surprised to see small delays, as it tries to find room and do tree maintenance operations.  I don't know what a good number is, and you'll probably get a different answer from anyone you ask.  I like keeping at least 50GB free, but often keep more, "just because".

So my personal recommendations for maximums are -

* ReiserFS, essentially read-only usage - 99%

* ReiserFS, essentially read-write usage - 93%

* XFS, essentially read-only usage - 20GB free

* XFS, essentially read-write usage - 50GB free

* BTRFS, essentially read-only usage - 20GB free

* BTRFS, essentially read-write usage - 50GB free

But personal preferences and usages come into play here.  For example, you fill a drive with a bunch of videos, and start another, with no plans to change anything on the first drive.  But a month later a sequel comes out, and you want to keep it together with its siblings on the first drive.  If you didn't keep some free space there, you can't.  So many of us will always keep an extra 50 to 100 GB free, "just in case".

First off thanks to everyone for replying. I can confirm that my real world "feel" for RFS slowdowns matches what RobJ summarized above.

I see no reason not to base discussion now on RobJ  recommendations:

* ReiserFS, essentially read-only usage - 99%

* ReiserFS, essentially read-write usage - 93%

* XFS, essentially read-only usage - 20GB free

* XFS, essentially read-write usage - 50GB free

* BTRFS, essentially read-only usage - 20GB free

* BTRFS, essentially read-write usage - 50GB free

as any difference between these and my personal recommendations would be within an error margin anyway.

So the first thing that catches my eye is based on the 93% recommendation for ReiserFS in read-write usage versus the 50GB recommendation for XFS just how superior in this respect XFS is. An 8TB drive which formats at approx 7.96TB of usable space, ReiserFS needs 507GB extra reservation per disk. In a full array that is a whopping 11.6TB of extra "wasted" space.

One areas we havent touched on is SSD which apart from being a different technology are much smaller and the flat numbers above for XFS could consume a much higher percentage of the disk than intended.

New wording to cover smaller disks and SSDs:

* ReiserFS, essentially read-only usage - 99%

* ReiserFS, essentially read-write usage - 93%

* XFS/BTRFS, essentially read-only usage - 2% or 20GB free (whichever is smaller)

* XFS/BTRFS, essentially read-write usage - 5% or 50GB free (whichever is smaller)