Re: SSDs - SandForce or not?

On 15/11/12 01:42, Shaun Thomas wrote:
> On 11/14/2012 01:11 AM, Toby Corkindale wrote:
>
>> I'm wondering which type of SSDs would be better for use with
>> PostgreSQL.

Hi Shaun,
thanks for your info. I should probably have made it clear that I was
curious to know how the compression stuff affected the situation, aside
from the other variables.

I'm aware of the other issues you've mentioned, but I'm sure it's
helpful for other people reading this list to see them.

You make a good point about the TOAST tables, I hadn't thought of that.
(My data is mostly numeric here though)

thanks,
Toby

> 1. While the controller may or may not have an impact, the presence of
> an on-board super-capacitor will have more. SSDs should be considered
> malignant devices that will go out of their way to destroy your data,
> unless they have one of these.
>
> 2. Workload on a compressible system like PG is generally dependent on
> your data sets. If you have lots of TOAST data, which is already
> compressed, you get no benefit. If your use case doesn't show a lot of
> random writes, optimizing for them is of questionable value.
>
> 3. SSDs also exist as effectively raw NVRAM, in the form of PCIe cards.
> These cards come in several varieties, and these days, can be mounted in
> external PCIe chassis in hot-swap bays much like more conventional drive
> enclosures. Some of these use a kernel-level driver over a proprietary
> controller, using neither Sandforce or anything else. They are also
> close to an order of magnitude faster than an SSD because they discard
> the SATA/SCSI bus entirely.
>
> 4. SSDs do have limited write cycles, and whether it's write leveling or
> drive compression to reduce writes on the actual NVRAM chips, if you
> honestly have a high write load, you're better off with whatever card
> reports the highest longevity of the relatively scarce write cycles per
> cell.
>
> 5. You're more likely to get performance improvements pursuing SLC
> (single layer chips) versus cheaper MLC (multi-layer) for writing,
> because the controller doesn't have to mask writes to the proper layer.
>
> Basically, there's way more involved here than Sandforce vs. Others. Or
> even Compressible vs. Not. SSDs are still a pretty Wild West kind of
> thing, and you've got a lot more variables to consider than with
> standard spindles.