PE4600 and Red Hat Questions Matt_Domsch at
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.

Matt Domsch
Sr. Software Engineer
Dell Linux Solutions
Linux on Dell mailing lists @
#1 US Linux Server provider for 2001! (IDC Mar 2002)

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