I am running defrag for the first time in a while... and my free space is dropping like crazy. Size Used Avail Use% Mounted on30T 28T 632G 98% /data When this started, I had over 1TB of free space. And it's going down now, about 1GB per minute. Any ideas what is happening? There's no one writing or adding data to the disks at all.

Let's review what CoW actually does: CoW simply allows multiple versions of a file to use only one copy of the datablocks they have in common. In a snapshot context, if a 300 MB video file (for example) is in both a snapshot and the main share, then all the video data is held in common. So both files share all the datablocks, and only 300 MB of data is actually on the disk. This will normally be contiguous. If you then edit a tagfield in the main file, then the datablock holding that tagfield has changed. So one datablock is different, all the others are still common. The file in the snapshot remains continguous. But in the main share, a new datablock is substituted for the original. The main share file is therefore fragmented into two (or possibly three) sections. Now you run a defrag on the main share. The only way to defrag this video file is to replicate all the shared blocks - taking 600 MB of total space, instead of the original 300 MB. Now the question which is unanswered: Why are so many GBs of new datablock usage showing up in your system when you have no snapshots? mdgm says "CoW can create them anyway" - he might well be right, but I don't see how. I think it is likely that you have snapshots that you can't see. This has happened to some users as a side-effect of software updates. I think you need to sort out if you have these hidden snapshots [or not] before you start making changes. What do you see with btrfs fi df /data/ShareA and btrfs fi df /data/ShareB Also, do you see any paths containing .snapshots with btrfs subvolume list /data

Also, there are no snapshots. just wanted to clarify that up front so that can't be the cause.

Sounds like some of your files probably use CoW. Do you (or did you) have bit-rot protection enabled on any of your shares? This is separate to CoW but we link enabling/disabling bit-rot protection to enabling/disabling CoW. You should free up space before running volume maintenance on a volume that is this full. Which firmware are you running?What does your initrd.log look like?

my initrd.log: 2014/01/15 14:52:32] Factory default initiated by button! [2014/01/15 14:52:47] Defaulting to X-RAID2 mode, RAID level 5 [2014/01/15 14:53:03] Factory default initiated on ReadyNASOS 6.0.4. [2014/01/15 14:49:34] Updated from ReadyNASOS 6.0.4 to 6.1.5. [2014/01/15 16:02:20] Updated from ReadyNASOS 6.1.5 to 6.1.6. [2014/04/14 05:09:27] Updated from ReadyNASOS 6.1.6 (1389750453) to 6.1.7 (1396977042). [2014/05/24 11:33:24] Updated from ReadyNASOS 6.1.7 (1396977042) to 6.1.8 (1398980083). [2014/10/28 16:39:30] Updated from ReadyNASOS 6.1.8 (1398980083) to 6.1.9 (1409791183). I have Bitrot Protection (Copy on Write) enabled on all shares. Not sure wha tyou mean by Bitrot being separate to CoW since its the same option. Firmware 6.4.2. What's the link between what I am seeing and bitrot?

They are not the same thing but we link enabling them/disabling them. Running a defrag won't uncow files. It will break the CoW link between current data and snapshots, but even if not using snapshots you will see an increase in volume usage if you are using CoW.

Running defrag, losing 1GB a minute

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joe_schmo
Tutor
Oct 13, 2016
Also, there are no snapshots. just wanted to clarify that up front so that can't be the cause.
- mdgm-ntgr
  NETGEAR Employee Retired
  Oct 13, 2016
  Sounds like some of your files probably use CoW.
  
  Do you (or did you) have bit-rot protection enabled on any of your shares? This is separate to CoW but we link enabling/disabling bit-rot protection to enabling/disabling CoW.
  
  You should free up space before running volume maintenance on a volume that is this full.
  
  Which firmware are you running?
  
  What does your initrd.log look like?
  - joe_schmo
    Tutor
    Oct 13, 2016
    my initrd.log:
    
    2014/01/15 14:52:32] Factory default initiated by button! [2014/01/15 14:52:47] Defaulting to X-RAID2 mode, RAID level 5 [2014/01/15 14:53:03] Factory default initiated on ReadyNASOS 6.0.4. [2014/01/15 14:49:34] Updated from ReadyNASOS 6.0.4 to 6.1.5. [2014/01/15 16:02:20] Updated from ReadyNASOS 6.1.5 to 6.1.6. [2014/04/14 05:09:27] Updated from ReadyNASOS 6.1.6 (1389750453) to 6.1.7 (1396977042). [2014/05/24 11:33:24] Updated from ReadyNASOS 6.1.7 (1396977042) to 6.1.8 (1398980083). [2014/10/28 16:39:30] Updated from ReadyNASOS 6.1.8 (1398980083) to 6.1.9 (1409791183).
    
    I have Bitrot Protection (Copy on Write) enabled on all shares. Not sure wha tyou mean by Bitrot being separate to CoW since its the same option.
    
    Firmware 6.4.2.
    
    What's the link between what I am seeing and bitrot?

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Running defrag, losing 1GB a minute

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