Shared Memory

[Henry Spencer]

On Tue, 1 Feb 2005, Lennart Sorensen wrote:
> ...There has been some work on linux to implement command
> reordering in the kernel to essentially emulate command queueing in
> software and simply make sure to keep track of where the drive is...

While I haven't looked at the Linux code lately, disk scheduling (the
original technical buzzphrase for this) has been done in Unix kernels for
decades; this is not some new experimental idea. 

As Lennart notes, since drives got smart enough to do their own bad-sector
remapping, the kernel doesn't have completely reliable knowledge of the
order of sectors.  And for some time now, its knowledge of the drive
geometry has been an approximation to the truth at best.  But it can still
do a respectable job... and it can maintain much longer queues than most
of the drives supported until recently. 

> ...The kernel can help more by simply doing
> read ahead by predicting what a program might want next in addition to
> what it just asked for...

Readahead too has been a standard feature of Unixoid systems for decades.

                                                          Henry Spencer
                                                       henry-lqW1N6Cllo0sV2N9l4h3zg at public.gmane.org

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