raid1 write performanc

Hello,

I recently migrated my system to btrfs on two drives (spinning) configured as raid1. When writing to this filesystem, the system shows some weird behaviour: it starts off with writing to both drives simultaneously for a few seconds, but after that it starts writing to the drives one after another: the system writes first to one drive while the second is idle and then it writes to the second drive while the first is idle.

During this time all IO to the filesystem seems to hang. This means the system is unusably unresponsive during large writes and the overall write speed averages out at about 30MB/s.

Any help is appreciated,
Henning

server ~ # uname -a
Linux server 2.6.38-rc2-git1 #1 SMP Mon Jan 24 21:27:58 CET 2011 x86_64 AMD Athlon(tm) 64 X2 Dual Core Processor 4200+ AuthenticAMD GNU/Linux

server ~ # btrfs fi df /mnt/pool/
Data, RAID1: total=832.00GB, used=801.43GB
System, RAID1: total=8.00MB, used=124.00KB
System: total=4.00MB, used=0.00
Metadata, RAID1: total=4.25GB, used=2.50GB

server ~ # btrfs fi show
Label: none uuid: 7676eb78-e411-4505-ac51-ccd12aa5a6b6

	Total devices 2 FS bytes used 803.94GB
	devid    1 size 931.28GB used 836.26GB path /dev/sda2
	devid    3 size 931.27GB used 836.26GB path /dev/sdb2