Re: Thoughts on using SSD

[Neil Brown <neilb@suse.de>]

On Thursday April 9, davidsen@tmr.com wrote:
> Neil Brown wrote:
> > On Thursday March 26, davidsen@tmr.com wrote:
> >   
> >> I'm building a fairly aggressive machine for both a backup host for 
> >> virtual machines and spare time development platform, compile engine and 
> >> testbed both. I want to get cost effective use from an SSD unit, and I 
> >> propose to use a 32GB unit as follows: for the root filesystem, 12GB, 
> >> which should hold all the usual root things, and 16GB for swap (12GB 
> >> RAM, and I want boot and/or hibernate to happen NOW). The remaining 
> >> space I think might be used for various high impact things, and one of 
> >> those with speeding raid.
> >>
> >> If I were to create a small raid device, raid1, made of the 4GB Ssd and 
> >> 4GB of SATA space, if I made the SATA write-mostly and write-behind, and 
> >> put the journal for my raid arrays (and bitmaps?) that seems likely to 
> >> provide a significant performance gain in small storage.
> >>
> >> Am I missing anything here? Is there an obvious drawback I'm missing?
> >>     
> >
> >
> > I'd probably just put the journal on the SSD and mount my ext3
> > filesystem data=journal
> >
> > That has a similar effect to raid1/write-behind in that data is
> > written to both but we only wait for the write to the SSD to
> > complete.  But as it is done at the filesystem level - and the
> > filesystem has a much better idea what it is doing - you would expect
> > to get much more efficient results.  e.g. less wasted memory, much
> > larger amount of data that is safe of SSD but still trickling out to
> > the HD.
> >   
> 
> But I think for raid in general you would benefit from having the bitmap 
> on SSD as well. In my dreams I also put the inodes on that SSD, and 
> everything runs 10x faster. Unfortunately no f/s seems to offer this.

Sounds like hierarchical storage management at a different level.
Keep the metadata on a fast device and allow the data to migrate on to
slow devices.
I'm sure we'll get there one day.... if only there were more hours in
the day :-)

NeilBrown