On Tue, 10 Sep 2002, Tom Lane wrote: > Curt Sampson <[EMAIL PROTECTED]> writes: > > Well, for the sequential reads, the readahead should be trigerred > > even when reading from a raw device. > > That strikes me as an unportable assumption.
Not only unportable: but false. :-) NetBSD, at least, does read-ahead only through the buffer cache. Thinking about it, you *can't* do read-ahead on a raw device, because you're not buffering. Doh! > Perhaps it's time to remind people that what we want to measure > is the performance seen by a C program issuing write() and read() > commands, transferring 8K at a time, on a regular Unix filesystem. Right. Which is what randread does, if you give it a file rather than a raw device. I'm actually just now working on some modifications for it that will let you work against a bunch of files, rather than just one, so it will very accurately emulate a postgres random read of blocks from a table. There are two other tricky things related to the behaviour, however: 1. The buffer cache. You really need to be working against your entire database, not just a few gigabytes of its data, or sample data. 2. Multiple users. You really want a mix of simultaneous accesses going on, with as many processes as you normally have users querying the database. These can probably both be taken care of with shell scripts, though. cjs -- Curt Sampson <[EMAIL PROTECTED]> +81 90 7737 2974 http://www.netbsd.org Don't you know, in this new Dark Age, we're all light. --XTC ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
