[OpenIndiana-discuss] Recommendations for fast storage

[Jim Klimov]

On 2013-04-16 19:17, Mehmet Erol Sanliturk wrote:
> I am not an expert of this subject , but with respect to my readings in
> some e-mails in different mailing lists and from some relevant pages in
> Wikipedia about SSD drives , the following points are mentioned about SSD
> disadvantages ( even for "Enterprise" labeled drives ) :
>

My awareness in the subject is of similar nature, but with different
results someplace... Here goes:

>
> SSD units are very vulnerable to power cuts during work up to complete
> failure which they can not be used any more to complete loss of data .

Yes, maybe, for some vendors. Information is scarce about which
ones are better in practical reliability, leading to requests for
info like this thread.

Still, some vendors make a living by selling expensive gear into
critical workloads, and are thought to perform well. One factor,
though not always a guarantee, of proper end-of-work in case of
a power-cut, is presence of either batteries/accumulators, or
capacitors, which power the device long enough for it to save its
caches, metadata, etc. Then the mileage varies how well who does it.

>
> MLC ( Multi-Level Cell ) SSD units have a short life time if they are
> continuously written ( they are more suitable to write once ( in a limited
> number of writes sense ) - read many )  .
>
> SLC ( Single-Level Cell ) SSD units have much more long life span , but
> they are expensive with respect to MLC SSD units .

I hear SLC are also faster due to more simple design. Price stems
from requirement to have more cells than MLC to implement the same
amount of storage bits. Also there are now some new designs like
eMLC which are young and "untested", but are said to have MLC price
and SLC reliability.

With decrease of sizes in technical process, diffusion and brownian
movement of atoms plays an increasingly greater role. Indeed, while
early SSDs boasted tens and hundreds of thousands of rewrite cycles,
now 5-10k is good. But faster.

>
> SSD units may fail due to write wearing in an unexpected time , making them
> very unreliable for mission critical works .

For this reason there is over-provisioning. The SSD firmware detects
unreliable chips and excludes them from use, relocating data onto
spare chips. Also there is wear-leveling, it is when the firmware
tries to make sure that all ships are utilized more or less equally
and on average the device lives longer. Basically, an SSD (unlike
a normal USB Flash key) implements a RAID over tens of chips with
intimate knowledge and diagnostic mechanisms over the storage pieces.

Overall, vendors now often rate their devices in gbytes of writes
in their lifetime, or in full rewrites of the device. Last year we
had a similar discussion on-list, regarding then-new Intel DC S3700
http://comments.gmane.org/gmane.os.solaris.opensolaris.zfs/50424
and it struck me that in practical terms they boasted "Endurance
Rating - 10 drive writes/day over 5 years". That is a lot for many
use-cases. They are also relatively pricey, at $2.5/gb linearly
from 100G to 800G devices (in a local webshop here).

>
> Due to the above points ( they may be wrong perhaps ) personally I would
> select revolving plate SAS disks and up to now I did not buy any SSD for
> these reasons .
>
> The above points are a possible disadvantages set for consideration .

They are not wrong in general, and there are any number of examples
where bad things do happen. But there are devices which are said to
successfully work around the fundamental drawbacks with some other
technology, such as firmware and capacitors and so on.

It is indeed not yet a subject and market to be careless with, by
taking just any device off the shelf and expecting it to perform
well and live long. Also it is beneficial to do some homework during
system configuration and reduce unnecessary writes to the SSDs - by
moving logs out of the rpool, disabling atime updates and so on.
There are things an SSD is good for, and some things HDDs are better
at (or are commonly thought to be) - i.e. price and longevity past
infant death toll, and the choice of components does depend on
expected system utilization as well as performance requirements
as well as how much you're ready to cash up for that.

All that said, I haven't yet touched an SSD so far, but mostly due
to financial reasons with both dayjob and home rigs...

//Jim