Re: Thoughts on using SSD

[Bill Davidsen <davidsen@tmr.com>]

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.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc

"You are disgraced professional losers. And by the way, give us our money back."
    - Representative Earl Pomeroy,  Democrat of North Dakota
on the A.I.G. executives who were paid bonuses  after a federal bailout.