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Re: Slow RMAN Backups

From: Naqi Mirza <>
Date: Fri, 18 Aug 2006 13:48:54 +0100 (BST)
Message-ID: <>

We have tried different combinations of filesperset and maxopenfiles (filesperset=60 maxopenfiles=60, filesperset=60 maxopenfiles=20 - various other combinations) - we have even disabled multiplexing setting maxopenfiles=1. For some reason the backup remains at a steady throughput of about 50mb/s. The write to tapes seem to be fine, it appears to be the read from the disks that seems to be the cause of the slow down. For example if we use the following settings:

filesperset=1 and maxopenfiles=1 (1 channel), we achieve a throughput of about 33mb/s. If we then use the same settngs of filesperset=1 and maxopenfiles=1 (2 channels), the total throughput becomes 50mb/s. However the throughput that was initially being achieved by 1 channel (33mb/s), drops by about 10mb/s.After this if the same setting is used and an additional channel is allocated the total throughput remains at about 50mb/s and the individual throughput for each of the channels drops.

There is a private network between the server and the tape library for backups. The tape library is an LTO3 with 3 drives available for the backup - each operating at 60mb/s.



Fuad Arshad <> wrote: Whats your filesperset.  whats the network infrastructure.
 are your direct attached to the tapes or goinf via/public/private lan.  have you monitored the tapes to see if you are consisitently streaming.  What generation of tape drives d o you have


I was wondering whether anyone has experienced poor backup times with oracle databases on RAID 5? We are using netbackup in conjunction with rman to backup a 1.5TB database that takes about 16 hours to complete. A host of rman parameters have been tried - combinations of filesperset and maxopenfiles - with no benefit seen. Our database uses RAID 5 spread over about a 100 disks. We have noticed that no matter what we do the maximum throughput for the backup is only 50mb/s. We are using an LTO3 tape library which can write at 60mb/s per tape drive. But no matter how many tape drives we allocate for use (we use 3 channels - 3 drives) we only get 50mb/s as the total throughput - resulting in a backup that is over 16 hours. What happens is, if one channel is allocated the throughput for that might be around 50mb/s, when a second channel is allocated, the total throughput remains around 50mb/s which is distributed over the two channels. Adding a third gives no benefit as the  50mb/s throughtput is then distributed over all three channels. I have monitored v$backup_aysnc_io and have noticed that there is for the most part poor read rates as seen by the effective_bytes_per_second for TYPE='INPUT'. From that I can only infer that the slower the read done by rman the slower the backup will be - I do not see any problem with respect to writes to the tape drives - they will write as fast as they recieve data. We have seen with a filesystem backup we are able to achieve a rate of about 100mb/s - although this type of backup is different to an rman backup there is quite a difference seen in the rate. Just wondering if there is someway to work out if the RAID configuration could be the issue here and whether the diskratio parameter might be a viable option to try. Thanks                    

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-- Received on Fri Aug 18 2006 - 07:48:54 CDT

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