Re: GBDE write performance really sucks

Maxim Sobolev wrote this message on Mon, Dec 13, 2004 at 21:22 +0200:
> Little more investigation revealed that the problem was due to disabled 
> write cache in ata(4). It would be interesting to compare FreeBSD 
> behaviour to behaviour of other operating systems in such situation, 
> since the drop of sequental writing performance in the case of 8KB 
> blocks and disabled write cache in FreeBSD is about 20x (from more than 
> 20MB/sec to merely 1MB/sec), which doesn't look reasonably to me.

I've heard (though not confirmed) that many hard disks use larger than
512byte blocks for on disk data..  If this is the case, when you turn
off write cacheing you maybe requiring the hard disk to do a read/write
of the same block significantly impacting performance...

Don't forget that doing purely sequental writes introduces a bandwidth
product into the mix..  If it always takes your hd 5ms to respond to a
command, and you only do 8KB/sec, then we end up with a max data rate of
1600KB.  With write caching disabled, you prevent the hd from optimizing
how it does the writes.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."