On Tuesday, 21 March 2000 at 9:29:56 -0800, Matthew Dillon wrote:
>>>
>>> I would think that track-caches and intelligent drives would gain
>>> much if not more of what clustering was designed to do gain.
>>
>> Hm. But I'd think that even with modern drives a smaller number of bigger
>> I/Os is preferable over lots of very small I/Os. Or have I missed the point?
>
> As long as you do not blow away the drive's cache with your big I/O's,
> and as long as you actually use all the returned data, it's definitely
> more efficient to issue larger I/O's.
>
> If you generate requests that are too large - say over 1/4 the size of
> the drive's cache, the drive will not be able to optimize parallel
> requests as well.
I think that in the majority of cases there's no need to transfer more
than requested. It could only apply to reads anyway, and the drive
cache probably has this data anyway. In RAID adapters, it seems to
almost always be due to poor firmware design. For regular files, it
might be an idea to set a flag to indicate whether read-ahead has any
hope of being useful (for example, on an ftp server the answer would
be "yes"; for index-sequential files or such the answer would normally
be "no".
Greg
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