Re: hdparm

[Alexander <vulture@abac.com>]

Hi...

Well, that explains a lot, thanks.

I'm not running RAID, this is just a single UDMA/33 disk. But... what
about that extra bandwidth helping with transferring data from the HDD's
own cache?

And I have just heard mention of solid-state HDDs. What are those?

Alex

On 20 Jul 1998, Gary L. Hennigan wrote:

> Date: 20 Jul 1998 09:06:15 -0600
> From: "Gary L. Hennigan" <glhenni@cs.sandia.gov>
> To: debian-user@lists.debian.org
> Subject: Re: hdparm
> Resent-Date: 20 Jul 1998 15:06:18 -0000
> Resent-From: debian-user@lists.debian.org
> Resent-cc: recipient list not shown: ;
> 
> Alexander <vulture@abac.com> writes:
> | Hi...
> | 
> | Umm:
> | 
> | /dev/hda:
> |  Timing buffer-cache reads:   64 MB in  1.61 seconds =39.75 MB/sec
> |  Timing buffered disk reads:  32 MB in  6.87 seconds = 4.66 MB/sec
> | 
> | Buffer-cache reads? Uh... explain that to me please, this particular UDMA
> | can't go past 33 MB/s.
> 
> That's just the cache Linux reserves in your RAM. That's why it's
> 39.75MB/s. The fastest single disk in existence is rated at about
> 20MB/s. So the disk you give results for above is getting roughly
> 4.66MB/s and you're getting about 40MB/s from the RAM cache.
> 
> | But I do believe I heard of a UDMA/66 or something like that. I'm not
> | using that here, though, so...
> 
> Yes, I remember hearing something about that too. Plus, there's U2W
> SCSI rated at 80MB/s. Again, for a single disk access the bandwidth,
> whether it's 33 (UDMA), 40 (UW SCSI), 66 (UDMA/66) or 80MB/s (U2W
> SCSI), is overkill. The fastest disks manufactured are currently the
> 10,000RPM drives, e.g., Seagate Cheetah, and their peak performance is
> 20MB/s, and that's peak, which means probably only when
> reading/writing data on the outter tracks would you ever get that
> rate. Of course you benefit from the extra bandwidth if you have
> multiple devices on that bus, say you have UW SCSI rated at 40MB/s,
> then you can run two of those Chetah's simultaneously without degraded 
> performance.
> 
> My point was that if you're benchmarking a disk and you get greater
> than 20MB/s you're seeing the results of caching, either in RAM or on
> the disk itself. Or you have some special set up, e.g., solid state
> disk (they still make these?) or a RAID.
> 
> Gary
> 
> 
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