Re: SSDs

On Wed, Apr 2, 2008 at 1:16 AM, James Mansion
<james(at)mansionfamily(dot)plus(dot)com> wrote:
> Tried harder to find info on the write cycles: found som CFs that claim
> 2million
> cycles, and found the Mtron SSDs which claim to have very advanced wear
> levelling and a suitably long lifetime as a result even with an
> assumption that
> the underlying flash can do 100k writes only.
>
> The 'consumer' MTrons are not shabby on the face of it and not too
> expensive,
> and the pro models even faster.
>
> But ... the spec pdf shows really hight performance for average access,
> stream
> read *and* write, random read ... and absolutely pants performance for
> random
> write. Like 130/s, for .5k and 4k writes.
>
> Its so pants it looks like a misprint and it doesn't seem to square with the
> review on tomshardware:
> http://www.tomshardware.com/2007/11/21/mtron_ssd_32_gb/page7.html
>
> Even there, the database IO rate does seem lower than you might hope,
> and this *might* be because the random reads are very very fast and the
> random writes ... aren't. Which is a shame, because that's exactly the
> bit I'd hope was fast.
>
> So, more work to do somewhere.

if flash ssd random write was as good as random read, a single flash
ssd could replace a stack of 15k disks in terms of iops (!).

unfortunately, the random write performance of flash SSD is indeed
grim. there are some technical reasons for this that are basically
fundamental tradeoffs in how flash works, and the electronic processes
involved. unfortunately even with 10% write 90% read workloads this
makes flash a non-starter for 'OLTP' systems (exactly the sort of
workloads you would want the super seek times).

a major contributing factor is that decades of optimization and
research have gone into disk based sytems which are pretty similar in
terms of read and write performance. since flash just behaves
differently, these optimizations

read this paper for a good explanation of this [pdf]:
http://tinyurl.com/357zux

my personal opinion is these problems will prove correctable due to
improvements in flash technology, improvement of filesystems and raid
controllers in terms of flash, and introduction of other non volatile
memory. so the ssd is coming...it's inevitable, just not as soon as
some of us had hoped.

merlin