Re: non volatile ram devices

["Ragnar Kjørstad" <linux-ide-arrays@ragnark.vestdata.no>]

On Wed, Dec 04, 2002 at 08:59:35PM +0100, Russell Coker wrote:
> I have some servers that are giving inadequate disk performance for Maildir 
> mail spools.  They are running kernel 2.4.19 (2.4.20 upgrade is planned) and 
> using ReiserFS for everything that's important.

One thing you might considder is replacing the reiserfs hash with a
maildir-specific hash. In my rather limited testing I found that it was
significantly faster; I think some tests gave 200-300% speed
improvement.

But, as I said, there was only limited testing. Don't go this route
unless you have the time to test it properly both for stability and
performance.


> What I am thinking of doing is using a kernel that supports data journalling 
> which should increase performance, but still probably won't give me enough.  
> So I am thinking of using an "external journal" (or using software RAID to 
> put the part of the partition containing the journal on a different device).
> 
> The device containing the journal would be something much faster than physical 
> media.

Even if the device is just a regular disk it should give you a real
performance boost. Depending on your RAID-setup, it may not be the
throughput, but the seeking back and forth between the journal and the
rest of the disk that kills performance. Having the journal on a
seperate disk solves that problem.


> Does anyone know of an affordable ($1000 or less) device that can survive 
> unexpected power outages of at least 24 hours duration, can commit a write in 
> less than 1ms, supports unlimited writes, and connects to a IDE or SCSI bus 
> (or PCI if there's a suitable Linux driver).

Did you check out Micro Memory Inc? (http://www.umem.com/) 
I think they have some PCI-cards (with linux-drivers) which may be
suitable for this. 

However, the main strength of flash/RAM devices is that you can do
random writes very fast. For a journal deice all access will be
sequential, so there may not be much advantage compared to using a
seperate disk for the journal? I've never tried, so I'm not sure exactly
how well it would work.

Is your server read- or write- bound? I've found that some mailservers
are IO-bound because of reads (I guess pop- and imap-servers that are
polling), and then the external journal is not likely to help.



-- 
Ragnar Kjørstad