Re: What is the best layer/device for a write-back cache based in nvram?

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Wed, 14 Sep 2016, Edgar Fuß wrote:

> > 2- In scattered writes contained in a same slice, it allows to reduce
> > the number of writes. With RAID 5/6 there is a advantage, the parity
> > is written only one time for several writes in the same slice, instead
> > of one time for every write in the same slice.
> > 3- It allows to consolidate several writes that takes the full length
> > of the stripe in one write, without reading the parity. This can be
> > the case for log structured file systems as LFS, and allows to use a
> > RAID 5/6 with the similar performance of a RAID-0.
> You ought to adjust youd slice size and FS block size then, I'd suppose.
> 
> I specifically don't get the LFS point. LFS writes in segments, which are 
> rather large. A segment should match a slice (or a number of them)
> I would suppose LFS to perform great on a RAIDframe. Isn't Manuel Bouyer 
> using this in production?
> 
> > 4- Faster synchronous writes.
> Y E S.
> This is the only point I fully aggree on. We've had severe problems with 
> brain-dead software (Firefox, Dropbox) performing tons of synchronous 4K 
> writes (on a bs=16K FFS) which nearly killed us until I wrote Dotcache 
> (http://www.math.uni-bonn.de/people/ef/dotcache) and we set XDG_CACHE_HOME 
> to point to local storage.

Hm...  Maybe what you need to do is make the LFS segment the same size as 
the RAID stripe, then mount LFS async so it only ever writes entire 
segments....

Eduardo