Re: support for external persistent cache

[Roberto Spadim <roberto@spadim.com.br>]

i think we could make it
but not a raid feature
a linux feature
for example

we could get ramdisk, nbd (network block device), ssd, hd, scsi, raid
and select what is first cache, and what is real value

after this device is created we could use it with raid...


but.... the problem persist
if we lost energy, we can´t confirm that ram memory is sync to disk,
just if we don´t use memory cache... in other words we will work with
the best cache disk speed)

since ram can get 32GB on big home computer
and ssd can get 1terabyte on big home computer
and hdd can get more than 1terabyte on big home computer

we just need time to sync 32GB from memory to ssd
what´s the ssd speed? 300mb/s? we need 106,6666666666667 seconds to
sync memory to ssd, can you buy a ups with 107 seconds of life with a
serial cable (usb maybe) that inform that we are without energy?





2011/1/19 Cory Coager <ccoager@gmail.com>:
> On Wed, Jan 19, 2011 at 02:44:31PM -0200, Roberto Spadim wrote:
>> yes, a jornaling but using ssd first and sata after
>> it?s not raid feature...
>> it?s per disk filesystem feature...
>> we could implement jornaling in raid too...
>> but?s more inteligent (easy) at application level (filesystem)
>> you could write first at network (if it?s faster than sata) and after
>> at sata (if it?s slower than network)
>>
>> it?s not a raid feature... it?s a per device feature got it?
>> maybe a device (not raid..) for a cache division on devices
>> for example
>>
>>
>> /dev/sda (sata 1terabyte 100mb/s)
>> /dev/sdb (ssd 1gigabyte 1000mb/s)
>>
>> /dev/cache_a (a mix of sata and ssd with sata size 1terabyte, and
>> mixed speed (memory, ssd, sata))
>>
>> cache_a device should know that
>> early reads/write should be writen to sdb
>> time in time it should sync at sda
>>
>> the same happens with memory (ram memory) but it?s volatille (diferent
>> than ssd that?s not volatille)
>>
>> /dev/sdb should be sync
>> /dev/sda should be async (since /dev/sdb make it safe to use async)
>>
>>
>> that?s you intention? i don?t know if linux have it, anyone know?
> I would use a physical ramdisk over a ssd but thats besides the point.
> They are still disk drives to the OS.
>
> Yes, I guess it would be similar to journaling but done in RAID
> itself.  I don't believe it exists yet, thats why I'm asking if
> we could add support for this?
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-- 
Roberto Spadim
Spadim Technology / SPAEmpresarial
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