more RAM + faster disk -> slower box?!

Paul Richards paul at freebsd-services.com
Thu Nov 29 15:19:50 GMT 2001


--On Thursday, November 29, 2001 13:24:48 +0000 Nik Clayton
<nik at freebsd.org> wrote:

> On Thu, Nov 29, 2001 at 12:34:38PM +0000, Ian Pallfreeman wrote:
>> I have a news server (INN, trad spool) which recently lost a disk, part
>> of a mirrored pair holding all news stuff except the articles (i.e.
>> binaries,  history, active file, overview). The articles are on a RAB
>> 3600 IDE<->SCSI RAID, which, frankly, sucks, and has been the cause of
>> much sorrow. The  replacement disk is a SCSI 160MB/s, replacing an 80 --
>> same size, same  geometry. And I took the opportunity to increase the
>> memory from 256MB to 768MB, in the hope that this might further
>> compensate for the bandwidth problem in the IDE RAID. 
> 
> I know that some older motherboards never cached more than the first
> 512MB of memory.  This was a particular problem with Windows, because it
> was loaded in to the top of memory.  If you had more than 512MB of RAM
> you suddenly discovered that your OS was running in non-cached memory,
> and everything got slower.
>
> FreeBSD is probably susceptible to the same problems (not sure where the
> OS loads, but given FreeBSD's aggressive use of memory you're going to
> be hitting that top 256MB very soon after boot).
> 
> I don't know whoch motherboards had this problem, but Google can
> probably tell you.  It might be worth ripping out 256MB and trying the
> test again.

Adding non-cacheable memory to a FreeBSD box should still makes things
faster, if you actually need to use that memory, since even uncached memory
is still a lot faster than swapping to disk.

Paul Richards
FreeBSD Services Ltd
http://www.freebsd-services.com




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