[ale] LAMP performance

[Greg Freemyer]

>> I wonder about SSDs.  They can only handle a fixed number of
>> transactions per lifetime.  They use wear-leveling to smooth it out,
>> but a 128KB erase block can typically only take 50,000 writes per
>> lifetime.
>>
>> Does anyone know if that ends up being a real world limitation or not?
>>
>> At a minimum I'd want to research that aspect.  But I suspect new
>> generation SSDs will handle it fine.
>>
>
> Current generation SSDs should do fine.  I've read that Intel's X25 is
> supposed to be able to do continuous writes for the period of the
> warranty before it will fail.  I don't know if the math actually
> supports that or not but the amount of real world writes you can put an
> X25 through are definitely huge.
>
> I think the biggest trick they use is the fact that 5-15% of the drive
> isn't actually addressable (depending on make/model).

The purpose of the spare 5-15% is purely for performance, not lifetime.

Every SSD write pulls a physical EB (erase block) off of the Free EB
queue and puts data on it.  Then a mapping table is updated so the SSD
knows where to read the logical address from.

The old physical EB is then pushed onto the bottom of the EB queue.
It has to be physically wiped before it is available to be pulled off
the top.  That can take some number of millisecs.  So if 100% of the
SSD were in use, the queue would be empty and the write operations
would be waiting for available EBs all the time.  The end result is
that a SSD is actually slower than a hard drive.

AIUI, most low cost thumb drives use the same technology but don't
perform block erasing in the background.  Thus their extremely low
performance.

SSDs like the Intel have the spare EBs sitting around so they can
erase the EBs in the background, thus attempting to eliminate the
erasing of blocks from being a bottleneck.

I don't know enough about how logical sectors are mapped to EBs to
calculate the lifetime writes achievable in a good SSD like an Intel.
It may depend on the exact number of sectors being written per write,
etc.

Greg