really slow RMAN backups

  • From: Steve Perry <sperry@xxxxxxxxxxx>
  • To: ORACLE-L <oracle-l@xxxxxxxxxxxxx>
  • Date: Mon, 21 Aug 2006 17:08:06 -0500

This was just passed to me, but I thought I'd check with the group to see if anyone else has experienced this slowness.

I don't have any experience with netapp and want to see if there are some known issues with it.

RMAN backups (2 tape channels) take forever on this system. forever means 20+ hours.

the view v$backup_sync_io shows the effective bytes per second at 2 or 3 MB per second. nothing above 5MB per second.
v$backup_async_io doesnt' show anything.

Setup.
500GB database on a netapp filer (40+ disks, don't know the model) with ASM
32-bit 10.2.0.1
2 - node RAC EE cluster
rhel3
2 cpu
1 GB swap
4GB ram
600 MB SGA (small and uses the automatic memory management)
flash recovery area is on
DG is setup for 2 different databases
mtu sizes of all NICs are set to 1500 (since it's netapp, they might prefer something else)
legato is the media manager

I looked at the init.ora settings and besides the small sga,
disk_asynch_io = false
filesystemio_option = directIO
large_pool_size = 52M

I don't know why they chose directio (1 dbwr) instead of async. they may not have anything to do with it, but it's the first time I saw them set on a RAC database.

I ran an awr report and "RMAN backup & recovery I/O" was the top waiter with an avg wait of 134 ms. the class is "system io".
other things are an index with 19 million get buffs during 2 hour snap shot.
I see a few slow access times 300ms avg. read time, but there are only 200 or so reads against it. Most of the access times are less than 20ms.
I don't know if the problem is contention with other jobs, config parameter or hardware.

I checked a similar system (db ver, 2 node rac, asm) that gets 80-90MB per second for it's backup.
it's on the SAN and uses async.
I haven't looked at the awr report from it.

any suggestions?
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