[CentOS] Re: Anaconda doesn't support raid10

Thu May 10 18:44:31 UTC 2007
Les Mikesell <lesmikesell at gmail.com>

Ross S. W. Walker wrote:

>> I've always wanted a dollars to dollars comparison instead of 
>> comparing 
>> single components, and I've always thought that a bunch of RAM could 
>> make up for slow disks in a lot of situations.  Has anyone 
>> done any sort 
>> of tests that would confirm whether a typical user would get better 
>> performance from spending that several hundred dollars 
>> premium for scsi 
>> on additional ram instead?  Obviously this will depend to a certain 
>> extend on the applications and how much having additional 
>> cache can help 
>> it, but unless you are continuously writing new data, most things can 
>> live in cache - especially for machines that run continuously.
> RAM will never make up for it cause user's are always accessing files
> that are just outside of cache in size, especially if you have a lot
> of files open and if the disks are slow then cache will starve to
> keep up.

I'm not convinced 'never' is the right answer here although you are of 
course right that cache can't solve all problems.  Most of the speed 
issues I see on general purpose machines are really from head contention 
where a hundred different applications and/or users each want the head 
to be in a different place at the same time and end up waiting for each 
other's seeks.  If some large percent of those requests can resolve from 
cache you speed up all the others.  It's a hard thing to benchmark in 
ways that match real world use, though.

> Always strive to get the best quality for the dollar even if quality
> costs more, because poor performance always makes IT skills look bad.

This isn't really a quality issue, it's about the tradeoff between a 
drive system that does somewhat better at actually handling lots of 
concurrent seek requests vs. cache to avoid the need to do many of those 
seeks at all.  For the cases where the cache works, it will be hundreds 
of times faster - where it doesn't, the slower drive might be tens of 
times slower.

> Better to scale down a project and use quality components then to use
> lesser quality components and end up with a solution that can't
> perform.

If you have an unlimited budget, you'd get both a scsi disk system with 
a lot of independent heads _and_ load the box with RAM.  If you don't, 
you may have to choose one or the other.

> SATA is good for it's size, data-warehousing, document imaging, etc.
> SCSI/SAS is good for it's performance, transactional systems, huge
> multi-user file access, latency sensitive data.

No argument there, but the biggest difference is in how well they deal 
with concurrent seek requests.   If you have to live with SATA due to 
the cost difference, letting the OS have some RAM for its intelligent 
cache mechanisms will sometimes help.  I just wish there were some 
benchmark test values that would help predict how much.

   Les Mikesell
     lesmikesell at gmail.com