Re: Forming drives together into a cache hierarchy

[Paul Richards <paul.richards@gmail.com>]

2009/9/2 Jochen Schulz <ml@well-adjusted.de>:
> Ron Johnson:
>> On 2009-09-02 05:30, Paul Richards wrote:
>>>
>>> Is there a way to combine a large slow drive with a fast small drive
>>> in such a way that the faster drive simply becomes a cache for the
>>> larger drive?
> -- snip
>> I'd think about selling the SSD and either buy more RAM or a SCSI
>> controller and a 10K RPM drive.
>
> While I would never recommend to sell an SSD (only to buy a larger one
> :)), I think buying more RAM is probably the way to go. Nothing beats
> the filesystem cache, not even an SSD.
>
> My recommendation: buy more RAM, install the OS to an SSD and copy the
> data most often used there as well. Put the rest on a regular disk and
> write a boot script to force the data you are interested in into the
> filesystem cache.
>
> If you cannot tell which data you are interested in in advance or if
> this subset of your data changes often, "caching" on an SSD is a hard
> problem to solve anyway.
>

I have to admit I'm slightly surprised by your response.  Caching has
massive benefit for main memory, so much so that we have several
layers of cache.  What is different about hard disks?  I don't see why
RAM (filesystem cache) <-> SSD <-> Rotating disk is such a bad idea..

An SSD cache has a number of benefits over a filesystem cache in RAM.
Price per GB will be much lower, and also it will persist over a
reboot.


-- 
Paul Richards