Re: SSD + RAID

On 02/22/2010 08:04 PM, Greg Smith wrote:
> Arjen van der Meijden wrote:
>> That's weird. Intel's SSD's didn't have a write cache afaik:
>> "I asked Intel about this and it turns out that the DRAM on the Intel
>> drive isn't used for user data because of the risk of data loss,
>> instead it is used as memory by the Intel SATA/flash controller for
>> deciding exactly where to write data (I'm assuming for the wear
>> leveling/reliability algorithms)."
>> http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3403&p=10
>
> Read further down:
>
> "Despite the presence of the external DRAM, both the Intel controller
> and the JMicron rely on internal buffers to cache accesses to the
> SSD...Intel's controller has a 256KB SRAM on-die."
>
> That's the problematic part: the Intel controllers have a volatile
> 256KB write cache stored deep inside the SSD controller, and issuing a
> standard SATA write cache flush command doesn't seem to clear it.
> Makes the drives troublesome for database use.

I had read the above when posted, and then looked up SRAM. SRAM seems to
suggest it will hold the data even after power loss, but only for a
period of time. As long as power can restore within a few minutes, it
seemed like this would be ok?

>> I can understand a SSD might do unexpected things when it loses power
>> all of a sudden. It will probably try to group writes to fill a
>> single block (and those blocks vary in size but are normally way
>> larger than those of a normal spinning disk, they are values like 256
>> or 512KB) and it might loose that "waiting until a full block can be
>> written"-data or perhaps it just couldn't complete a full block-write
>> due to the power failure.
>> Although that behavior isn't really what you want, it would be
>> incorrect to blame write caching for the behavior if the device
>> doesn't even have a write cache ;)
>
> If you write data and that write call returns before the data hits
> disk, it's a write cache, period. And if that write cache loses its
> contents if power is lost, it's a volatile write cache that can cause
> database corruption. The fact that the one on the Intel devices is
> very small, basically just dealing with the block chunking behavior
> you describe, doesn't change either of those facts.
>

The SRAM seems to suggest that it does not necessarily lose its contents
if power is lost - it just doesn't say how long you have to plug it back
in. Isn't this similar to a battery-backed cache or capacitor-backed cache?

I'd love to have a better guarantee - but is SRAM really such a bad model?

Cheers,
mark