Disk Performance on Cluster
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Posted: 17 Feb 2004
Question: I was wondering if anybody can tell me what kind of disk performance I can expect if I setup a cluster with two nodes and a server as an iSCSI target for those nodes.
I was thinking of using three HP DL360G3 servers and a dedicated Gigabit network for iSCSI.
How much slower would this be compared to direct attached storage or a SAN? Can I use this for disc applications like iFolder and home directories? Any advice would be welcome.
Answer: One expert told us that a fibre fabric will work at 2GB in each direction with no attenuation. The GB LAN has a theoretical maximum bandwidth of some 700MB.
The performance of the LAN is however probably better than the disc performance from any IDE RAID, and when looking at a SCSI system you will need at least 9 disks (including two parity) to flood the GB LAN.
Anyone else care to chime in on this topic? Don't be shy.
This question (disk performance in general, and specifically on a cluster) is a common one. The information is out there, but it is necessary to dig up component information, and then piece the puzzle together. I have a set of papers I'm putting together on the subject, but here are some short answers:
Raw gigabit performance depends on implementation details (copper gigabit isn't true gigabit, BTW) but remember that it is a full duplex connection and traditional read/write ratios are 3:1. This means that raw throughput over fiber connections could reach 1 GBit/s /8 bits, or 120 Mbytes/s out, 40 Mbytes/s in. Protocol and file system overhead will clearly reduce this maximum, even if the disk channel could deliver it.
Multiple FC standards exist. The current is 2 GBit/s, vendors are wrangling over 4 and 10 GB for the next step - and the original FC was, if memory serves 800 Mbits/s.
Server performance is nearly always limited by the disk channel - specifically too few drives. While drive performance for sequential access has continued to improve random i/o access (the common access pattern in multi user, multitasking environments) has stayed nearly flat. Worse, most systems use RAID 5, which provides maximum usable disk space at the expense of write performance. In an extreme example, a 3 drive RAID 5 system could experience small write performance as poor as 1-1.5 Mbytes/s.
Editor's Note: Randy has graciously offered to share his papers with us when he's got them finished. We're looking forward to that!
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