>
> On Tue, 25 Mar 2008, mx wrote:
>
> > The atom unit of flash is page(512~2048byte typically). Page are
> > organized into blocks, typically of 32 or 64 pages. All read write and
> > write operations happen at page granularity, but erase operations happen
> > at block granularity.
>
> You made a subtle switch here I wanted to emphasise. Your original
> message suggested flash is an increasingly important storage mechanism
> because flash devices like SSD drives are going to be more popular in the
> future; that is true. However, what you're describing is something more
> like how flash is used in raw embedded systems applications. The kinds of
> SSD drives that are becoming popular for database use abstract away all of
> this low-level block mess and hide it with approaches like sophisticated
> write-leveling algorithms.
>
Maybe I gives too many detailed about raw flash devices.
In fact, what I want to show is the asymmetric speed of read and write.
Any flash devices including SSD have such a characteristic.
For a sumsung 64G SSD PATA IDE 2.5,maximum Squential Read is 57MB/s,
while maximum sequential Read is 38MB/s according the product datasheet.
Anyway, in the eyes performance of outside， write is more expensive than read.
Some strategy of trade read for write may be considered.
So, the asymmetric speed of read and write make it is still valuable
to do some work on SSD.
>You don't (and possibly can't) even know what
> the underlying structure is like. And even if you did, the fact that
> there's a always a regular operating system and filesystem underneath
> PostgreSQL writes will make it undertain the writes are only touching the
> tiny portion of flash you want to target anyway. They may write a whole
> OS block regardless.
Yeah, you're right. This is the most confused thing. I wish my thesis
work is independent of low level flash device. But it's very hard in
fact, just as what you said.
--
Have a good day;-)
Best Regards,
Xiao Meng
━━━━━━━━━━━━━━━━━━━━━━━━━
Data and Knowledge Engineering Research Center,CS&T
Harbin Institute of Technology, Harbin, China
Gtalk: mx.cogito@gmail.com
MSN: cnEnder@live.com
Blog: http://xiaomeng.yo2.cn
