TCP problems

My testing for a cheap NAS solution has progressed to the option that costs the most money, Windows 2008 running KernSafe's iStorage. As it happens, it works really well when the iSCSI initiator is Windows but Linux clients don't really want to talk to it. Windows: 30-50 MB/s. Linux: 3-5 MB/s. Biiiig difference there.

Looking at packets I'm noticing a similar pattern on the wire to one I'd seen before. Back when I was troubleshooting exactly why NetWare backups to DataProtector were horrible I came across this problem. It seems that TCP Windowing is fundamentally broken between Server 2008 and NetWare which leads to really bad throughputs, which in turn is very bad for half TB backups. The receiving server seemed to feel the need to ACK after every two packets, which in turn really slowed things down. And that's what the Linux clients are doing for iSCSI to Server 2008.

It has to be something affecting basic TCP services but not complex protocols. Using smbclient to upload a 4GB DVD iso runs at 50MB/s but the iSCSI throughput on the same client is a piddly 3-5MB/s. I'm sure some kind of tuning on either side might be able to jar things loose, heaven knows Linux 2.6.31 is a heck of a lot more current on TCP settings than NetWare 6.5 SP8 is. I just haven't found it yet.

Conversely, Server 2008 talking to a Linux iSCSI client works at line speed pretty much. I'm testing this for completeness's sake. We need something that can serve up to 30TB via both iSCSI and SMB. My findings aren't fully complete yet, but in general:
  • OpenFiler: GREAT iSCSI host, completely blows for SMB in our environment.
  • OpenSolaris: Great iSCSI host, just can't convince the kernel-mode CIFS to join our domain. Also, worst-of-breed random I/O performance.
  • OpenFiler + Windows: OpenFiler for iSCSI, Windows (mounting an iSCSI share) for SMB. Should work GREAT. Current best-best for the future.
  • OpenSolaris + Windows: As previous option, but I/O problems make it less attractive.
  • Windows + KernSafe: GREAT SMB performance, solid iSCSI for Windows hosts. Linux hosts will take lots of tuning (perhaps, or it could be intractable).


Howdy hey!

Just a (former) ResTek nerd saying hi. Happened to see your username on Server Fault (darn, you're prolific!), and had a strange feeling of "I know that name from somewhere..."

Hope all is well, glad to hear that OpenFiler testing looks so good. I've been investigating it as well for iSCSI use.

Replace your Windows target with something more usable. Like DataCore SANmelody or StarWind iSCSI target. They both have cheap versions ($199 for SANmelody Lite and I think StarWind guys have free version at all). And you're not going to suffer from performance degradation.