Hi all,
my RAID array is filling up, so this bug started to effect me as well.
My NAS is running from a 40GB SSD drive.

Linux CSNAS 2.6.38-8-server #42-Ubuntu SMP Mon Apr 11 03:49:04 UTC 2011 x86_64 
x86_64 x86_64 GNU/Linux
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=11.04
DISTRIB_CODENAME=natty
DISTRIB_DESCRIPTION="Ubuntu 11.04"

I have a RAID6 setup, 16x2TB disks using an Adaptec 51645 card. It is a single 
EXT4 volume:
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1            11518469088 10420881676 1097587412  91% /mnt/raid6

Mount params:
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# RAID6 - backups
/dev/sda1       /mnt/raid6  ext4  suid,dev,exec,nodelalloc  0       0

I was able to transfer data with a sustained transfer speed of 96MB/sec from my 
PC to it via LAN (which is nearly the full Gigabit bandwidth) using Samba 
(client: Windows 7 x64 SP1).
Now that I passed the 90% mark, the write speeds have dropped to a very low 
rate and I get random speeds in the range of 1MB/sec to 14MB/sec.
At quite a few occasions Windows reports 'disk full' and the transfer is 
aborted; this is most annoying with large files (size>10GB) because I simply 
cannot transfer these anymore (they time out eventually).

Dunno if it's related, but since this is not a system disk, I have
reduced the reserved space to 0 (meaning that I should be able to
utilize the remaining space just the same).

Read speeds (no write):
cs@CSNAS:~$ sudo hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads: 1208 MB in  3.00 seconds = 402.63 MB/sec

Read speeds (write):
cs@CSNAS:~$ sudo hdparm -t /dev/sda
/dev/sda:
 Timing buffered disk reads: 314 MB in  3.29 seconds =  95.51 MB/sec

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https://bugs.launchpad.net/bugs/791992

Title:
  btrfs in 2.6.38 extremely slow (when near full?)

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