> -----Original Message----- > From: Mohan, Ross [mailto:[EMAIL PROTECTED] > Sent: Thursday, April 14, 2005 1:30 PM > To: pgsql-performance@postgresql.org > Subject: Re: [PERFORM] How to improve db performance with $7K? > > Greg Stark wrote: > > > > Kevin Brown <[EMAIL PROTECTED]> writes: > > > > > Greg Stark wrote: > > > > > > > I think you're being misled by analyzing the write case. > > > > > > > > Consider the read case. When a user process requests a block > > > > and that read makes its way down to the driver level, the > > > > driver can't just put it aside and wait until it's convenient. > > > > It has to go ahead and issue the read right away. > > > > > > Well, strictly speaking it doesn't *have* to. It could delay > > > for a couple of milliseconds to see if other requests come in, > > > and then issue the read if none do. If there are already other > > > requests being fulfilled, then it'll schedule the request in > > > question just like the rest. > > > > But then the cure is worse than the disease. You're basically > > describing exactly what does happen anyways, only you're > > delaying more requests than necessary. That intervening time > > isn't really idle, it's filled with all the requests that > > were delayed during the previous large seek... > > [...] > > [...] > 1) some SATA drives (just type II, I think?) have a "Phase Zero" > implementation of Tagged Command Queueing (the special sauce > for SCSI). > [...] > Largely b/c of 2 and 3, multi-initiator SCSI RAID'ed drives > are likely to whomp SATA II drives for a while yet (read: a > year or two) in multiuser PostGres applications.
I would say it depends on the OS. What Kevin is describing sounds just like the Anticipatory I/O Scheduler in Linux 2.6: http://www.linuxjournal.com/article/6931 For certain application contexts, it looks like a big win. Not entirely sure if Postgres is one of them, though. If SCSI beats SATA, it sounds like it will be mostly due to better seek times. __ David B. Held Software Engineer/Array Services Group 200 14th Ave. East, Sartell, MN 56377 320.534.3637 320.253.7800 800.752.8129 ---------------------------(end of broadcast)--------------------------- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
