Lars wrote:
Below is a quote from the Pliant datasheet: "No Write Cache: Pliant EFDs deliver outstanding write performance without any dependence on write cache and thus does not use battery/supercap."
I liked the article The Register wrote about them, with the headline "Pliant's SSDs are awesome, says Pliant". Of course they do. Check out the write benchmark figures in the information review at http://oliveraaltonen.com/2010/09/29/preliminary-benchmark-results-of-the-pliant-ssd-drives/ to see how badly performance suffers on their design from those decisions. The Fusion I/O devices get nearly an order of magnitude more write IOPS in those tests.
As far as I've been able to tell, what Pliant does is just push writes out all the time without waiting for them to be aligned with block sizes, followed by cleaning up the wreckage later via their internal automatic maintenance ASICs (it's sort of an always on TRIM implementation if I'm guessing right). That has significant limitations both in regards to total write speed as well as device longevity. For a database, I'd much rather have a supercap and get ultimate write performance without those downsides. Depends on the read/write ratio though; I could see a heavily read-biased system work well with their approach. Of course, a heavily read-based system would be better served by having a ton of RAM instead in most cases.
Could be worst though--they could be misleading about the whole topic of write durability like Intel is. I consider claiming high performance when you don't always really have it, what Pliant is doing here, to be a much lesser sin than losing data at random and not being clear about when that can happen. I'd like FusionIO to put a big "expect your server to be down for many minutes after a power interruption" warning on their drives, too, while I'm wishing for complete vendor transparency here.
-- Greg Smith 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
