On Sat, Apr 11, 2009 at 2:41 AM, er krishna <erkris...@gmail.com> wrote:
> You are using dd to test it . I will suggest that ratehr using dd, hdparam
> , iostat go for some tools like fio,bonnie,iozone & iometer. Please once do
> your experiment with any of this tool and let us know the results.
All of those use filesystems and not all applications use filesystems -
Oracle and other database can make use of block devices directly - ditto for
windows and others. Furthermore, layering a filesystem on it severely
complicates things because now you have (what seems like) 100's more
variables to contend with - as if testing the block layer wasn't complicated
enough.
I *would* suggest that if dd is to be used, that 'conv=fdatasync' be used -
this tells dd to issue an fdatasync(2) call before closing - this forces the
block layer to write out all *cached* data for this file. Otherwise, dd may
have written 1G and the block layer is caching 300MB or more - but writing
to the cache is writing to memory and therefore the performance values can
be rather exaggerated.
> I don't think iSCSI can give better throughput, because it has tcp/ip
> overhead.
It sure can because TCP/IP has very sophisticated mechanisms (window
scaling, pluggable algorithms) to handle packet loss, fairness, stability,
etc.. I have seen nearly perfect TCP/IP throughput on gig-e links (well
north of 120MB/s) and have never gotten anywhere near that with AoE. I get
/substantially/ better I/O with nbd (a TCP/IP based network block device on
Linux) than I do with AoE for just this reason. This is not to knock AoE!! I
feel that AoE is a perfectly appropriate, even ideal solution for the set of
problems that it solves, but to say that iSCSI could not possibly be faster
than AoE is to forget about the decades of work that has gone into making
TCP/IP scale and perform.
--
Jon
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