Re: Is my Oracle Server issuing more IO than it can handle

From: OKH <>
Date: Tue, 7 Dec 2010 17:57:14 +0100
Message-Id: <>


well, for a wannabe your quite good analyzing. I'd glad to have some wannabes like you..

first: the high free buffer waits indicates that you're reading a lot from disk. As the dbw usually writes asynchronously I suspect that it also writes with low priority. So this could explain the high average write times. I'd first check the buffer cache size to lower free buffer waits and than check again


Felix Castillo Sanchez

Am 07.12.2010 um 17:35 schrieb Oracle Dba Wannabe <>:

> Hi All, this is a single instance database (non asm). I see the following events from awr (1 hour snapshot - however hourly snapshots after this show the same trend with respect to wait events):
> Event Waits Time(s) Avg Wait(ms) % Total Call Time Wait Class
> free buffer waits 17,926,869 193,146 11 67.7 Configuration
> log file switch (private strand flush incomplete) 41,550 30,538 735 10.7 Configuration
> log file sync 211,675 25,156 119 8.8 Commit
> buffer busy waits 42,093 23,218 552 8.1 Concurrency
> db file parallel write 376 14,274 37,963 5.0 System I/O
> I know that db file parallel write only contributes to 5% of the total call time - but its avg wait time looks extremely poor - that and the fact that free buffer waits appear at top indicate that there's a db writer issue (db_writer_processes=4) - which leads me to believe perhaps its the IO subsystem. Now the storage team report there is nothing up with the storage. I was hoping someone could help with the following questions:
> 1. Is there someway from awr that I can determine that the Oracle server is issuing more IO than the storage system can handle for example:
> Physical reads: 954.74 16.68
> Physical writes: 418.89 7.32
> Phy Reads + Phy Writes = 1372 IOPS
> Can I then say that if each disk can do 100 IOPS, that the storage system should at least have 13 Disks? (13x100 IOPS)? Or is that an over simplification?
> 2. Interestingly this DB server was moved onto a new box with a different storage and the issue is no longer observed there.
> Transactions Per Second on old box = 57.22
> Transactions Per Second on new box = 225
> Phy Reads + Phy Writes for New Box, are slightly under half compared to the old box:
> Physical reads: 243.02 1.08
> Physical writes: 564.62 2.51
> That said, the redo size per second on the new box is twice that of the old box (7mb/s : 3mb/s). The buffer cache and db writer processes are the same on both boxes.
> Other than the storage aspect of things, I'm thinking (and will check) whether theres an o.s/kernel misconfiguration w.r.t to async io, etc that might be causing this on the old box.
> Appreciate any thoughts on 1 or/and 2
> Thanks

Received on Tue Dec 07 2010 - 10:57:14 CST

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