Re: RAID0 - cache

[NeilBrown <neilb@suse.de>]

On Wed, 19 Jan 2011 18:13:23 -0200 Roberto Spadim <roberto@spadim.com.br>
wrote:

> Hi guys, from a long email i will start this one: (the last one wasn´t send)
> 
> think about this:
> 
> /dev/sda (sata disk 1000gb size, latency of 1ms to move head, 100mb/s
> to write/read)
> /dev/sdb (ssd 1gb size, latency of <100us to find what flash and
> position, 500mb/s to write/read)
> ram memory (8gb size, with 1gb not used)
> /dev/mdxxx (the new device i´m calling it as raid0-cache)
> 
> how raid0-cache work?
> 
> 
> how to create raid?
> tell what the sequencial read cache estrategy:
> memory(tell fixed memory size) - /dev/sdb - /dev/sda - another device
> (maybe just 2 is suficiente /dev/sdb and /dev/sda)...
> 
> 
> how to read?
> first, read from the first read device (memory)
> if information not there -> read from second read device (/dev/sdb),
> update information at first read device, return the read value
> if information not there -> read from third read device (/dev/sda),
> update information at second and first read device, return the read
> value
> 
> how to write?
> flush space from first read device to second read device
> write to first read device, return OK
> 
> background thread:
> sync from first write device (memory) to second write device (/dev/sdb)
> flush from second write device (/dev/sdb) to third write device (/dev/sda)
> 
> startup:
> using cache:
>  flush everythink from first write device (memory) to second write
> device (/dev/sdb)
>  flush everythink from second write device (/dev/sdb) to second write
> device (/dev/sda)
>  since memory is volatille, at startup it can´be flushed...
> without cache:
>  make first write device unusable (memory)
>  make second write device unusable
>  here we can have write problems, since a wrong power off, can lost
> information from cache (first and second write device)
> 
> 
> 
> anyidea who can make it?

"Hey, I've got this clever idea, I wish someone would implement it" almost
always results in ... nothing.
People with the ability to implement are much more interested in implementing
their own ideas.

But see also "bcache".

NeilBrown

> thanks
> --
> Roberto Spadim
> Spadim Technology / SPAEmpresarial
> --
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