Boaz Harrosh wrote:
From my tests, the block layer will, very fast, send you chained commands. if you set it to 255 you'll see two chained fragments. In iscsi we have 4096 and it is very good for performance. However since you are pre-allocating the memory per can_queue commands, this can get big, and the memory overhead can be an impact on performance.
I did repeated read tests with dd now, with a 3.5" 750G SATA/ FireWire 800 disk and with a 2.5" 250G IDE/ FireWire 400 disk, each with either FireWire stack. I used dd with and without iflag=direct, just because I don't have a clue how to capture the influence of .sg_tablesize best.
Tests with .sg_tablesize = 128, 255, and 5000 all yield the same transfer rates. So I guess I simply leave it at the current SG_ALL for now.
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