it is I/O scheduling.
i have a system with a large amount of RAM.
it has both 15K RPM SCSI disks (off a symbios controller) and some bog-slow
IDE/ATA disks which the system decides to use PIO for rather than DMA. (i
don't use them for anything other than bootup so don't really care about it
deciding to use PIO..).
a copy to/from the 15K RPM SCSI disks doesn't show any performance problems.
a copy to/from the PIO-based IDE disks has the same effect -- 20/30 seconds
of no interactiveness -- even a "vmstat 1" *stops* for 20-30 seconds while
200+MB of buffer-cache data gets written out to disk.
i'm guessing that:
(a) the i/o scheduler isn't taking into account "disk speed" and thus
show it more effectively than fast-disks
(b) its isolated to somewhere in the IDE drivers
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