Re: SSD Drives - Mailing list pgsql-general

On Fri, Apr 4, 2014 at 11:04 AM, Steve Crawford
<scrawford@pinpointresearch.com> wrote:
> On 04/03/2014 12:44 PM, Brent Wood wrote:
>
> Hi David,

My take:

> Does the RAID 1 array give any performance benefits over a single drive? I'd
> guess that writes may be slower, reads may be faster (if balanced) but data
> security is improved.

Probably not so much for SSD drives. Read and write performance are
very unbalanced in SSD and RAID1 doesn't help with writes.

> I've been looking into upgrading to SSD and wondering about RAID and where
> to apply $$$ as well. In particular I'm curious about any real-world
> PostgreSQL-oriented performance and data-protections advice in the following
> areas:
>
> 1. With SSDs being orders of magnitude faster than spinning media, when does
> the RAID controller rather than the storage become the bottleneck?

SSD (at least the good ones) are maybe order of magnitude faster on
writes.  Can be less or more depending on the application write
particulars.  SSD are 2-3 orders faster for reads.

> 2. Do I need both BBU on the RAID *and* capacitor on the SSD or just on one?
> Which one? I'm suspecting capacitor on the SSD and write-through on the
> RAID.

You need both. The capacitor protects the drive, the BBU protects the
raid controller.

> 2. Current thoughts on hardware vs. software RAID - especially since many of
> the current SSD solutions plug straight into the bus.

IMNSHO, software raid is a better bet.  The advantages are compelling:
Cost, TRIM support, etc. and the SSD drives do not benefit as much
from the write cache.   But hardware controllers offer very fast burst
write performance which is nice.

> 3. Potential issues or conflicts with SSD-specific requirements like TRIM.

TRIM is not essential but does help.  Pretty much all hardware raid
controllers do not support TRIM.  I've been waiting for a controller
that manages TRIM and other SSD stuff (like consolidated wear
leveling) across an entire array but so far nothing has really
materialized.  If it does happen it will probably come from intel.

> 4. Manufacturers, models or technologies to seek out or avoid.

Avoid consumer grade/enthusiast stuff, and anything that does not have
a capacitor.  Intel offerings tend to be the benchmark.

> 5. At what point do we consider the RAID controller an additional SPOF that
> decreases instead of increases reliability?
>
> 6. Thoughts on "best bang for the buck?" For example, am I better off
> dropping the RAID cards and additional drives and instead adding another
> standby server?

This is going to depend a lot on write patterns.  If you don't do much
writing, you can gear up accordingly.  For all around performance, the
S3700 (2.5$/gb) IMO held the crown for most of 2013 and I think is
still the one to buy.  The s3500 (1.25$/gb) came out and also looks
like a pretty good deal, and there are some decent competitors (600
pro for example).  If you're willing to spend more, there are a lot of
other options.  I don't think it's reasonable to spend less for a
write heavy application.

merlin