Re: Crash in __btrfs_reserve_extent

[Josef Bacik <josef@redhat.com>]

On Thu, Aug 06, 2009 at 10:53:24AM +0200, Tore Anderson wrote:
> * Tore Anderson
> 
> > I have btrfs on my root filesystem and at the time of the crash I
> > taring together some files from a NFS filesystem onto it.  There was
> > plenty of free space on the btrfs filesystem.
> 
> Some more info - the file system is acting really strange after a
> reboot.  A lot of messages like these are printed to /var/log/messages:
> 
> no space left, need 4096, 0 delalloc bytes, 16577376256 bytes_used, 0
> bytes_reserved, 0 bytes_pinned, 0 bytes_readonly, 69816320 may
> use16647192576 total
> 
> Many commands gives the error message "no space left on device".
> Curiously enough /var/log/messages is stored on the very same file
> system, and new lines appears in it, so it appears those writes are
> avoiding the ENOSPC somehow.
> 
> Anyway, there's plenty of free space on the file system:
> 
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/mapper/vg_echo-lv_root  228G   16G  212G   7% /
> 
> It has never been resized, by the way.  I found that this issue had been
> mentioned on the mailing list by Hugo Mills (Cc-ed) before after all -
> apologies for posting a dupe:
> 
> http://thread.gmane.org/gmane.comp.file-systems.btrfs/2694
> 
> Is this bug fixed in later versions of btrfs or is it still unresolved,
> I wonder?  If the latter, I'll be glad to help out with any debugging
> info you might need before I try to fix it (or re-install).
>

This is one of the gotchas of btrfs, there is not proper ENOSPC handling, just a
few things in place that are a bit conservative to make sure you don't panic the
box.  Btrfs has seperate zones used for data and metadata, and these chunks are
allocated in 1gb chunks, so  you have 212gb of space thats allowed for data use,
and 16gb thats allowed for metadata use.  By default every 12 (or it may be 8, i
forget) chunks we allocate for data, we allocate 1 for metadata, which ends up
with like 8% of the disk being used for metadata.

Now in your case you can run btrfsctl -b and it will re-balance the space on the
drive, and it may give you more space back.  It could also possible panic the
box, so make sure you are all backed up :).  Thanks,

Josef