On Monday 27 October 2003 14:40, Gordon Henderson wrote:

On Mon, 27 Oct 2003, Hermann Himmelbauer wrote:

quoted

Well - I have an old Dual P-II-266 System with an onboard SCSI-Controller
with 3 Ultra SCSI-disks connected, building a RAID5. I did a simple Test
with "hdparm -tT" to provide you with numbers:

/dev/sdb:
 Timing buffer-cache reads:   128 MB in  1.46 seconds = 87.67 MB/sec
 Timing buffered disk reads:  64 MB in  5.07 seconds = 12.62 MB/sec

It's possibly the "old Dual P-II-266" that may be slowing things down
here.

I also thought this at first. But looking at the system load with e.g. "top" 
or a graphic CPU-monitor the CPU load is only ~ 15%. When reading from a 
single drive, the load is higher, something like 25%. Moreover the most 
CPU-hungry application is the "hdparm"-Utility. The raid5d uses only ~ 2%.

On one of my systems: (Dual Athlon 2.4 with 2 Promise PCI IDE cards and 4
drives):
/dev/hdg6:
 Timing buffer-cache reads:   128 MB in  0.48 seconds =266.67 MB/sec
 Timing buffered disk reads:  64 MB in  1.49 seconds = 42.95 MB/sec
/dev/md4:
 Timing buffer-cache reads:   128 MB in  0.50 seconds =256.00 MB/sec
 Timing buffered disk reads:  64 MB in  0.97 seconds = 65.98 MB/sec

On another server (Dual PIII/Xeon 700MHz with SCSI drives)
/dev/sdc6:
 Timing buffer-cache reads:   128 MB in  0.59 seconds =216.95 MB/sec
 Timing buffered disk reads:  64 MB in  3.39 seconds = 18.88 MB/sec
/dev/md4:
 Timing buffer-cache reads:   128 MB in  0.57 seconds =224.56 MB/sec
 Timing buffered disk reads:  64 MB in  0.75 seconds = 85.33 MB/sec

That's interesting: both systems are 4-disk RAID5 arrays. The first gains ~50% 
read performance, the second gains ~ 470% (!) - probably there is caching 
involved.

Probably hdparm is not the best tool for benchmarking RAIDs...

So my advice is that if you want speed, be prepared to spend the £££ to
get that speed (and I don't consider these servers particularly fast, but
they are fast enough for my application which is NFS & Samba serving a
small company of engineers (software/hardware) via a single 100MB/sec
Ethernet interface).

Well, 5MB/s are o.k. for my server and my application - anyway it's 
interesting why this happens.

		Best Regards,
		Hermann

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