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 _______________________________________________ OpenIndiana-discuss mailing list OpenIndiana-discuss@openindiana.org http://openindiana.org/mailman/listinfo/openindiana-discuss