Re: Finding the Bottleneck
On Friday 08 June 2001 16:26, Jason Lim wrote:
> This statement makes me wonder:
> "Also even the slowest part of a 45G drive will be twice as fast as the
> fastest part of a 15G drive."
>
> Are you sure? I never heard this before... might be a 1% difference
There is a huge difference. I have tested many drives, I have not tested
the IBM 15G drives, but the IBM 10G and 13G drives are quite slow, I
typically see speeds of around 10MB/s compared to >30MB/s for the fast
part of a 45G drive! One of the main ways that modern drives are getting
better performance is by having more sectors per track. If you have
twice the number of sectors per track and the same rotational speed then
the maximum bandwidth is doubled.
> there, but twice as fast? The 15G runs at 7200rpms, and I assume the
> 45G also does unless its a 10000rpm unit (but I don't think there are
> any 1000rpm units for ata interfaces, only scsi, right?) The cache on
> the 15G is 2Mb, which I assume is the same on the 45G disk.
Of course it will depend on other factors too, the version of the kernel
(newer kernels give better results), the motherboard, etc. Download my
Bonnie++ package and try the zcav program that is included...
> "That an average of over 11 messages per second. Not bad with such
> tiny hardware."
>
> Well, it doesn't seem to come to raw hardware performance here. From
> "top" output, the 500mhz CPU isn't loaded and swap is not touched at
> all (totally in ram). Many small-mid range servers don't run Raid
> configurations. However, it does appear that a Raid setup will help
> greatly in this case, even with the lower end hardware specs, don't you
> think?
Definately. You should be able to get 4 times the performance by getting
new drives and using RAID-1.
> As soon as you clarify the first point, I think we'll be looking far
> more closely at Raid solutions (either software or cheap hardware....
> which would be better.... if we go with software the bottleneck might
> then be shifted to the CPU?!)
RAID-1 involves writes going to two drives at the same time, which is
exactly double the amount of CPU power for the system calls for writing
the blocks of data. The writing of the data should take less CPU power
than the file system overhead, and that should be less than the
application uses to decide what to write. Almost nothing multiplied by
two isn't much...
For reads RAID-1 takes no more CPU power than a single drive, as you only
read from a single drive.
RAID-5 however is quite different.
--
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