Re: How to improve db performance with $7K?

От: Kevin Brown
Тема: Re: How to improve db performance with $7K?
Дата: ,
Msg-id: 20050416005350.GF19518@filer
(см: обсуждение, исходный текст)
Ответ на: Re: How to improve db performance with $7K?  (Tom Lane)
Список: pgsql-performance

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How to improve db performance with $7K?  (Steve Poe, )
 Re: How to improve db performance with $7K?  (Tom Lane, )
  Re: How to improve db performance with $7K?  (Steve Poe, )
 Re: How to improve db performance with $7K?  (Will LaShell, )
 Re: How to improve db performance with $7K?  ("Joshua D. Drake", )
  Re: How to improve db performance with $7K?  (Steve Poe, )
   Re: How to improve db performance with $7K?  (Bjoern Metzdorf, )
    Re: How to improve db performance with $7K?  (Josh Berkus, )
     Re: How to improve db performance with $7K?  (Steve Poe, )
      Re: How to improve db performance with $7K?  (Cott Lang, )
       Re: How to improve db performance with $7K?  (Steve Poe, )
        Re: How to improve db performance with $7K?  (Cott Lang, )
         Follow-Up: How to improve db performance with $7K?  (Steve Poe, )
          Re: Follow-Up: How to improve db performance with $7K?  (Thomas F.O'Connell, )
           Re: Follow-Up: How to improve db performance with $7K?  (Kevin Brown, )
            Re: Follow-Up: How to improve db performance with $7K?  ("Steinar H. Gunderson", )
            Re: Follow-Up: How to improve db performance with $7K?  (Thomas F.O'Connell, )
          Re: Follow-Up: How to improve db performance with $7K?  (Vivek Khera, )
           Re: Follow-Up: How to improve db performance with $7K?  (Will LaShell, )
     Re: How to improve db performance with $7K?  (Steve Poe, )
    Re: Follow-Up: How to improve db performance with $7K?  (PFC, )
     Re: Follow-Up: How to improve db performance with $7K?  ("Steinar H. Gunderson", )
   Re: How to improve db performance with $7K?  (PFC, )
    Re: How to improve db performance with $7K?  (Dave Cramer, )
     Re: How to improve db performance with $7K?  (Greg Stark, )
     Re: How to improve db performance with $7K?  (Alex Turner, )
      Re: How to improve db performance with $7K?  (Steve Poe, )
       Re: How to improve db performance with $7K?  (Alex Turner, )
        Re: How to improve db performance with $7K?  (Vivek Khera, )
         Re: How to improve db performance with $7K?  (Alex Turner, )
        Re: How to improve db performance with $7K?  (William Yu, )
         Re: How to improve db performance with $7K?  (Alex Turner, )
          Re: How to improve db performance with $7K?  (William Yu, )
           Re: How to improve db performance with $7K?  (Alex Turner, )
            Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
             Re: How to improve db performance with $7K?  (Alex Turner, )
              Re: How to improve db performance with $7K?  (Alex Turner, )
               Re: How to improve db performance with $7K?  (Alex Turner, )
              Re: How to improve db performance with $7K?  (Greg Stark, )
               Re: How to improve db performance with $7K?  (Alex Turner, )
               Re: How to improve db performance with $7K?  (Tom Lane, )
                Re: How to improve db performance with $7K?  (Kevin Brown, )
                 Re: How to improve db performance with $7K?  (Greg Stark, )
                  Re: How to improve db performance with $7K?  (Kevin Brown, )
                   Re: How to improve db performance with $7K?  (Tom Lane, )
                    Re: How to improve db performance with $7K?  (Kevin Brown, )
                     Re: How to improve db performance with $7K?  (Alex Turner, )
                      Re: How to improve db performance with $7K?  (PFC, )
                     Re: How to improve db performance with $7K?  (Tom Lane, )
                      Re: How to improve db performance with $7K?  (Kevin Brown, )
                       Re: How to improve db performance with $7K?  (Tom Lane, )
                        Re: How to improve db performance with $7K?  (Kevin Brown, )
                      Re: How to improve db performance with $7K?  (Greg Stark, )
                     Re: How to improve db performance with $7K?  (Vivek Khera, )
                      Re: How to improve db performance with $7K?  ("Joshua D. Drake", )
                       Re: How to improve db performance with $7K?  (Vivek Khera, )
                      Re: How to improve db performance with $7K?  (Kevin Brown, )
                    Re: How to improve db performance with $7K?  (Matthew Nuzum, )
                     Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
                   Re: How to improve db performance with $7K?  (Rosser Schwarz, )
                    Re: How to improve db performance with $7K?  (Kevin Brown, )
                   Re: How to improve db performance with $7K?  (Greg Stark, )
                   Re: How to improve db performance with $7K?  (Bruce Momjian, )
                    Re: How to improve db performance with $7K?  (Alex Turner, )
                     Re: How to improve db performance with $7K?  (Alvaro Herrera, )
                     Re: How to improve db performance with $7K?  (Bruce Momjian, )
              Re: How to improve db performance with $7K?  ("Douglas J. Trainor", )
               Re: How to improve db performance with $7K?  (, )
                Re: How to improve db performance with $7K?  (Alex Turner, )
                 Re: How to improve db performance with $7K?  (, )
            Re: How to improve db performance with $7K?  ("Douglas J. Trainor", )
            Re: How to improve db performance with $7K?  (PFC, )
             Re: How to improve db performance with $7K?  (Alan Stange, )
    Re: How to improve db performance with $7K?  (Dave Cramer, )
 Re: How to improve db performance with $7K?  ("Alexander Kirpa", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
 Re: How to improve db performance with $7K?  ("Matthew Nuzum", )
  Re: How to improve db performance with $7K?  (Greg Stark, )
   Re: How to improve db performance with $7K?  (PFC, )
  Re: How to improve db performance with $7K?  (Tom Lane, )
 Re: How to improve db performance with $7K?  ("Dave Held", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
  Re: How to improve db performance with $7K?  (Steve Poe, )
   Re: How to improve db performance with $7K?  ("Joshua D. Drake", )
   Re: How to improve db performance with $7K?  (William Yu, )
    Re: How to improve db performance with $7K?  (Greg Stark, )
     Re: How to improve db performance with $7K?  (Alex Turner, )
      Re: How to improve db performance with $7K?  (Greg Stark, )
       Re: How to improve db performance with $7K?  (Alex Turner, )
        Re: How to improve db performance with $7K?  (Jacques Caron, )
         Re: How to improve db performance with $7K?  (Alex Turner, )
          Re: How to improve db performance with $7K?  (Alex Turner, )
           Re: How to improve db performance with $7K?  (Jacques Caron, )
            Re: How to improve db performance with $7K?  (Alex Turner, )
          Re: How to improve db performance with $7K?  (Jacques Caron, )
         Re: How to improve db performance with $7K?  (Greg Stark, )
        Re: How to improve db performance with $7K?  (Alan Stange, )
        Re: How to improve db performance with $7K?  (John A Meinel, )
         Re: How to improve db performance with $7K?  (Alex Turner, )
          Re: How to improve db performance with $7K?  ("Joshua D. Drake", )
           Re: How to improve db performance with $7K?  (Alex Turner, )
        Re: How to improve db performance with $7K?  (Steve Poe, )
         Re: How to improve db performance with $7K?  (Alex Turner, )
     Re: How to improve db performance with $7K?  (Jacques Caron, )
      Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
     Re: How to improve db performance with $7K?  (William Yu, )
 Re: How to improve db performance with $7K?  ("Dave Held", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
 Re: How to improve db performance with $7K?  ("Dave Held", )
  Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
  Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
   Re: How to improve db performance with $7K?  (Alex Turner, )
 Re: How to improve db performance with $7K?  ("Dave Held", )
  Re: How to improve db performance with $7K?  (Alex Turner, )
   Re: How to improve db performance with $7K?  (William Yu, )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
  Re: How to improve db performance with $7K?  (Bruce Momjian, )
   Re: How to improve db performance with $7K?  (, )
    Re: How to improve db performance with $7K?  ("Jim C. Nasby", )
     What to do with 6 disks?  (Jeff Frost, )
      Re: What to do with 6 disks?  ("Jim C. Nasby", )
      Re: What to do with 6 disks?  (Josh Berkus, )
       Re: Opteron vs Xeon (Was: What to do with 6 disks?)  (Jeff Frost, )
        Re: Opteron vs Xeon (Was: What to do with 6 disks?)  ("J. Andrew Rogers", )
         Re: Opteron vs Xeon (Was: What to do with 6 disks?)  (Jeff Frost, )
          Re: Opteron vs Xeon (Was: What to do with 6 disks?)  (William Yu, )
        Re: Opteron vs Xeon (Was: What to do with 6 disks?)  (Vivek Khera, )
       Re: What to do with 6 disks?  (Vivek Khera, )
      Re: What to do with 6 disks?  (William Yu, )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
  Re: How to improve db performance with $7K?  (Bruce Momjian, )
  Re: How to improve db performance with $7K?  (Dawid Kuroczko, )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )
 Re: How to improve db performance with $7K?  ("Dave Held", )
 Re: How to improve db performance with $7K?  ("Mohan, Ross", )

Tom Lane wrote:
> Kevin Brown <> writes:
> > In the case of pure random reads, you'll end up having to wait an
> > average of half of a rotation before beginning the read.
>
> You're assuming the conclusion.  The above is true if the disk is handed
> one request at a time by a kernel that doesn't have any low-level timing
> information.  If there are multiple random requests on the same track,
> the drive has an opportunity to do better than that --- if it's got all
> the requests in hand.

True, but see below.  Actually, I suspect what matters is if they're
on the same cylinder (which may be what you're talking about here).
And in the above, I was assuming randomly distributed single-sector
reads.  In that situation, we can't generically know what the
probability that more than one will appear on the same cylinder
without knowing something about the drive geometry.


That said, most modern drives have tens of thousands of cylinders (the
Seagate ST380011a, an 80 gigabyte drive, has 94,600 tracks per inch
according to its datasheet), but much, much smaller queue lengths
(tens of entries, hundreds at most, I'd expect.  Hard data on this
would be appreciated).  For purely random reads, the probability that
two or more requests in the queue happen to be in the same cylinder is
going to be quite small.


--
Kevin Brown                          


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