Re: hdparm
Alexander <vulture@abac.com> writes:
| 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?
Yes, the extra bandwidth can help there. But, for the majority of
drives that cache is less than 1MB, and maybe 4MB if you have an A/V
disk. Not a whole lot, but significant if you're working with lots of
small files. Also, I'm pretty sure hdparam trys to defeat the cache on
the disk.
| And I have just heard mention of solid-state HDDs. What are those?
They're just huge banks of RAM made to look like a disk. They're
specialty items and you won't see a lot of them, if any. Plus, with
the performance of disks getting into the 20MB/s range they're
probably even rarer these days. On the other hand RAM prices are so
low...Who knows? I've never seen one myself but have seen them
advertised in high-end hardware magazines. They are generally VERY
expensive.
Gary
| On 20 Jul 1998, Gary L. Hennigan wrote:
| > 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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