PE4600 and Red Hat Questions
Matt_Domsch at Dell.com
Tue Jun 25 08:04:01 CDT 2002
> How many MB/sec did you get with 128KB vs. 256KB vs. 512KB
> for sequential read/write vs. random read/write?
At some point increasing the size doesn't matter so much.
1) at 128KB and above, it *all* looks sequential to the disk.
2) you're really limited by the number of scatter/gather entries the
controller supports (64 in the case of the PERC3 series), and the 4KB page
size - you're going to max out at 256KB per I/O to the controller itself.
Memory is nearly perfectly fragmented, so you generally won't get two 4K
pages in a single sg entry.
3) It *may* help from user-space to ask for larger blocks because you suffer
fewer user->kernel->user syscalls. But underneath, the kernel's got to wait
on disk, which is hugely slower than the CPU.
Sr. Software Engineer
Dell Linux Solutions www.dell.com/linux
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