FAQ

On 7/26/13 7:32 AM, Tom Lane wrote:
Greg Smith <greg@2ndquadrant.com> writes:
On 7/26/13 5:59 AM, Hannu Krosing wrote:
Well, SSD disks do it in the way proposed by didier (AFAIK), by putting
"random"
fs pages on one large disk page and having an extra index layer for
resolving
random-to-sequential ordering.
If your solution to avoiding random writes now is to do sequential ones
into a buffer, you'll pay for it by having more expensive random reads
later.
What I'd point out is that that is exactly what WAL does for us, ie
convert a bunch of random writes into sequential writes. But sooner or
later you have to put the data where it belongs.
FWIW, at RICon East there was someone from Seagate that gave a presentation. One of his points is that even spinning rust is moving to the point where the drive itself has to do some kind of write log. He notes that modern filesystems do the same thing, and the overlap is probably stupid (I pointed out that the most degenerate case is the logging database on the logging filesystem on the logging drive...)

It'd be interesting for Postgres to work with drive manufacturers to study ways to get rid of the extra layers of stupid...
--
Jim C. Nasby, Data Architect jim@nasby.net
512.569.9461 (cell) http://jim.nasby.net

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