Re: Completely un-tuned Postgresql benchmark results: SSD vs desktop HDD

On Aug 7, 2010, at 11:49 PM, Michael March wrote:

SSD's actually vary quite a bit with typical postgres benchmark workloads.

You mean various SSDs from different vendors? Or are you saying the same SSD model might vary in performance from drive to drive?

Model to model (more specifically, controller chip to controller chip -- i.e. most 'Indilinx Barefoot' controller based SSD's perform similar).

Many of them also do not guarantee data that has been sync'd will not be lost if power fails (most hard drives with a sane OS and file system do).

What feature does an SSD need to have to insure that sync'd data is indeed written to the SSD in the case of power loss?

Either properly flush to storage when the OS / File sytem asks for it (most SSD's don't, most Hard Drives do), or have a supercapacitor to flush data on power loss.

The former can be achieved by turning off the write cache on some drives (such as Intel's X25-M and -E), but hurts performance.

Also, the amount of data at risk in a power loss varies between drives. For Intel's drives, its a small chunk of data ( < 256K). For some other drives, the cache can be over 30MB of outstanding writes.
For some workloads this is acceptable -- not every application is doing financial transactions. Not every part of the system needs to be on an SSD either -- the WAL, and various table spaces can all have different data integrity and performance requirements.

On Aug 7, 2010, at 4:47 PM, Michael March wrote:

If anyone is interested I just completed a series of benchmarks of stock Postgresql running on a normal HDD vs a SSD.

If you don't want to read the post, the summary is that SSDs are 5 to 7 times faster than a 7200RPM HDD drive under a pgbench load.

http://it-blog.5amsolutions.com/2010/08/performance-of-postgresql-ssd-vs.html

Is this what everyone else is seeing?

Thanks!