ext4, barrier, md/RAID1 and write cache

[Daniel Pocock <daniel@pocock.com.au>]

I've been having some NFS performance issues, and have been
experimenting with the server filesystem (ext4) to see if that is a factor.

The setup is like this:

(Debian 6, kernel 2.6.39)
2x SATA drive (NCQ, 32MB cache, no hardware RAID)
md RAID1
LVM
ext4

a) If I use data=ordered,barrier=1 and `hdparm -W 1' on the drive, I
observe write performance over NFS of 1MB/sec (unpacking a big source
tarball)

b) If I use data=writeback,barrier=0 and `hdparm -W 1' on the drive, I
observe write performance over NFS of 10MB/sec

c) If I just use the async option on NFS, I observe up to 30MB/sec

I believe (b) and (c) are not considered safe against filesystem
corruption, so I can't use them in practice.

Can anyone suggest where I should direct my efforts to lift performance?
 E.g.

- does SCSI work better with barriers, will buying SCSI drives just
solve the problem using config (a)?

- should I do away with md RAID and consider btrfs which does RAID1
within the filesystem itself?

- or must I just use option (b) but make it safer with battery-backed
write cache?

- or is there any md or lvm issue that can be tuned or fixed by
upgrading the kernel?