Re: Hardware vs Software RAID

On Thu, Jun 26, 2008 at 10:14 AM, Matthew Wakeling <matthew(at)flymine(dot)org> wrote:
> On Thu, 26 Jun 2008, Vivek Khera wrote:
>>>
>>> Anyone have experience with IDE, SATA, or SAS-connected flash devices
>>> like the Samsung MCBQE32G5MPP-0VA? I mean, it seems lovely - 32GB, at a
>>> transfer rate of 100MB/s, and doesn't degrade much in performance when
>>> writing small random blocks. But what's it actually like, and is it
>>> reliable?
>>
>> None of these manufacturers rates these drives for massive amounts of
>> writes. They're sold as suitable for laptop/desktop use, which normally is
>> not a heavy wear and tear operation like a DB. Once they claim suitability
>> for this purpose, be sure that I and a lot of others will dive into it to
>> see how well it really works. Until then, it will just be an expensive
>> brick-making experiment, I'm sure.
>
> It claims a MTBF of 2,000,000 hours, but no further reliability information
> seems forthcoming. I thought the idea that flash couldn't cope with many
> writes was no longer true these days?

What's mainly happened is a great increase in storage capacity has
allowed flash based devices to spread their writes out over so many
cells that the time it takes to overwrite all the cells enough to get
dead ones is measured in much longer intervals. Instead of dieing in
weeks or months, they'll now die, for most work loads, in years or
more.

However, I've tested a few less expensive solid state storage and for
some transactional loads it was much faster, but then for things like
report queries scanning whole tables they were factors slower than a
sw RAID-10 array of just 4 spinning disks. But pg_bench was quite
snappy using the solid state storage for pg_xlog.