Re: [PATCH] speed up SATA

[Eric D. Mudama]

On Mon, Mar 29 at 13:36, Pavel Machek wrote:
> 32MB is one second if everything goes okay... That will be horrible for latency, right?

It'll be somewhere between half a second and 1.5 seconds, depending on
how old your hard drive is, all other things being removed from the
equation.  The media rate of modern IDE drives is about 60MB/s right
now, and going up slightly soon.  The top SCSI drives are around
85MB/s if I remember correctly.

On average, 1s is about right.

It becomes a tradeoff between a big hit now and a lot of little hits
over time.  If you don't expect to have any idle time soon, you're
probably better off flushing as big a chunk as possible to free RAM
for the upcoming/continuous workload.  If you think you might have
idle time, the lower MB/s of smaller requests will keep you more
granular and improve latency, even though overall work completed per
unit time will be slightly lower.

For a bursty access pattern, you're probably best off with small
requests that fit within the drive's cache, but I don't have any sort
of measurable data on this.

Most of the OS testing that we're subjected to doesn't attempt large
requests.

--eric

--
Eric D. Mudama
edmudama@xxxxxxxxxxxxxxxxxxxxx

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