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Home -> Community -> Mailing Lists -> Oracle-L -> Re: Fw: oraperf comment
Thanks for your comments Tim. The reason I brought this up is that the "conventional wisdom" is, well, conventional. I like your logic, but have always lived on the other side of the tracks on this. I'll say one thing for oraperf, they got my attention. The oraperf writer used the exclusive:
"Never split index and data files to different sets of disks."
I wonder if they were making an overstatement in an effort to dispell "conventional wisdom". A statement about i/o balancing may be more appropriate.
BTW, some of my ts actually tend to operate in the opposite fashion from your generalized argument, but perhaps your argument holds true on the opposite side of the coin, but I still find "Never" tough to swallow:
Av Av Av Av Buffer Av Buf
Reads Reads/s Rd(ms) Blks/Rd Writes Writes/s Waits Wt(ms)
-------------- ------- --------- ------- ------------ -------- ---------- ------
TACPLUS_BINDEX
TACPLUS_BDATA
4,342 0 12.4 1.0 1,615 0 0 0.0
Never say never!
On Tue, Oct 22, 2002 at 07:14:46AM -0800, Tim Gorman wrote:
> ...resending, as the original send encountered some kind of "locking problem" at fatcity...
>
> ----- Original Message -----
> To: ORACLE-L_at_fatcity.com
> Sent: Tuesday, October 22, 2002 6:35 AM
>
>
> Why? What are the chances of precisely that scenario happening, as opposed to Oracle doing concurrent I/O to tables for both users A and B? Or to indexes for both users A and B simultaneously?
>
> Splitting tables and indexes into separate tablespaces makes sense, but mainly for recovery purposes. This has little to do with the placement of the datafiles of those tablespaces on devices (non-RAID or RAID).
>
> Generally, indexes tend to cache extremely well in Oracle (because they are more compact and because of the nature of the I/O), so they usually don't get as much physical I/O as tables. Check V$FILESTAT on a busy application to prove it for yourself...
>
> After seeing this performance data, why would you place a datafile/tablespace which only gets a small amount of I/O on one device while placing a much busier datafile/tablespace onto another device, just because one contains indexes and the other tables?
>
> Please think in terms of I/O counts, not poorly-conceived but oft-repeated "conventional wisdom". Keep indexes and tables segregated to different tablespaces, but for decisions on placement of datafiles upon devices, use empirical performance data only.
> ----- Original Message -----
> From: Yechiel Adar
> To: Multiple recipients of list ORACLE-L
> Sent: Tuesday, October 22, 2002 3:43 AM
> Subject: Re: oraperf comment
>
>
> Hello Tim
>
> I beg to differ. Without raid it is better to put indexes and tables on different disks and controllers.
> This way Oracle can do I/O to a table for user A while doing I/O to the index for user B.
>
> It is better if you can find the high I/O areas of the database and split them across disks, but as a rule of thumb splitting indexes and tables make sense (again - when you work without raid).
>
> Yechiel Adar
> Mehish
> ----- Original Message -----
> From: Tim Gorman
> To: Multiple recipients of list ORACLE-L
> Sent: Tuesday, October 22, 2002 12:39 AM
> Subject: Re: oraperf comment
>
>
> Ray,
>
> I don't know exactly what was intended with the comment, but I agree with your interpretation.
>
> ---
>
> As far as any other reasons for the comment...
>
> <RANT>
> In terms of myths that have persisted with Oracle over the years, the idea that some performance benefit exists from I/O parallelism due to separating tables and indexes to different devices has been especially persistent. I've even heard it described as "conventional wisdom". As a matter of fact, there is no possibility for "parallelism" benefits on indexed I/O operations. Never has been; might never be (though "never" is a long time)...
> </RANT>
>
> The reason is that navigating a B*Tree index structure is inherently sequential. Think about it -- first you have to access the "root" block. Looking inside the contents of the "root" directs you to the next "branch" or "leaf" block in the index B*Tree structure. You cannot seek for the next block in parallel; you've got to look inside one block in order to know what block to access next. Then, once you've accessed down to the final "leaf" block, reading its contents tells you which row in the table to access. If you are doing a "range scan" operation, then you have to go back to the index "leaf" block in order to find the next table row to access.
>
> The name of the wait-event for this type of I/O (a.k.a. "db file sequential read", a.k.a. single-block random-access read) also suggests this "sequentialiality" (is that a word?). Jeff Holt wrote a great paper on the reasons for the apparent mis-naming of the wait-events "db file sequential read" and "db file scattered read" -- I'm sure that it is downloadable from http://www.hotsos.com. Even when "asynchronous I/O" is available and configured, indexed I/O operations are still essentially synchronous (and non-parallel)...
>
> There is a possibility of some form of "parallelization" in "range-scan" operations, but there is no evidence that this is happening. For example, while performing an indexed range-scan, if we wanted to read a batch of index entries from the index "leaf blocks" and submit a list of I/O requests for data blocks on the corresponding table, we could do so. However, when I've performed "truss" operations on an Oracle server process performing such a range-scan operation (at least through Oracle8i), I've not seen this happening. Purely generic "read()" operations, one at a time, sequentially...
>
> ---
>
> The only real advantages of separating tables from indexes into different tablespaces are:
> a.. different recoverability requirements
> a.. indexes can be rebuilt instead of restored
> b.. data (tables and clusters) must be restored -- cannot be "rebuilt" from anything
> b.. different types of I/O requests
> a.. indexes are predominantly accessed using single-block, random read I/O (i.e. UNIQUE scans, RANGE scans, FULL scans)
> a.. relatively seldom are accessed with multi-block sequentially-accessed read I/O (i.e. FAST FULL scans)
> b.. while tables are often accessed with a mix of the two types of I/O, depending on the application
> a.. OLTP usually has heavier single-block, random read I/O due to heavy use of indexes
> b.. DW usually has heavier multi-block, sequentially-accessed read I/O due to heavy use of FULL table scans
> c.. may be advantages from this in Oracle9i where different blocksizes are possible for different tablespaces
> These last points are related to performance, but not in the sense that the mythical "conventional wisdom" dictates...
>
> Hope this helps...
>
> -Tim
>
> ----- Original Message -----
> From: "Ray Stell" <stellr_at_cns.vt.edu>
> To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> Sent: Monday, October 21, 2002 2:43 PM
> Subject: oraperf comment
>
>
> >
> > An recent oraperf report included the comment: "Never split index
> > and data files to different sets of disks." It goes on to state that
> > striping is better. If the system in question does not have
> > raid support, wouldn't it be better to split the index and data across
> > spindles? That would make the word "Never" inappropriate here? Maybe
> > this is their way of saying don't use old technology. Is there some
> > other reason I am missing?
> > ===============================================================
> > Ray Stell stellr_at_vt.edu (540) 231-4109 KE4TJC 28^D
> > --
> > Please see the official ORACLE-L FAQ: http://www.orafaq.com
> > --
> > Author: Ray Stell
> > INET: stellr_at_cns.vt.edu
> >
> > Fat City Network Services -- 858-538-5051 http://www.fatcity.com
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-- =============================================================== Ray Stell stellr_at_vt.edu (540) 231-4109 KE4TJC 28^D -- Please see the official ORACLE-L FAQ: http://www.orafaq.com -- Author: Ray Stell INET: stellr_at_cns.vt.edu Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: ListGuru_at_fatcity.com (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB ORACLE-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).Received on Tue Oct 22 2002 - 11:48:59 CDT
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