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SIZE_DATA_BUFFERS... Mix of SAS&SCSI.

Created: 28 Jan 2013 | 5 comments

Hi guys, I've just recently added a LTO3 SCSI to my NBU server because I needed the ability to read older tapes, whilst doing so I thought I might as well put it into the pool so I can utilise it for backups, the server has a SAS Quantum i40 with an LTO5, and the SCSI ArcVault 24 LTO3, I've had this issue before when trying to use my old LTO4 on a different NBU env, where the settings were tweaked and the buffers ramped up to improve performance.

But from what a consultant told me, SAS ignores the buffer sizes and just maxes out the bandwidth regardless? So if I were to reduce the buffers to accomodate the SCSI, it shouldn't have a knock on effect with the SAS and cure the below error?

 

1/24/2013 5:01:40 PM - Error bptm(pid=1836) The tape device at index -1 has a maximum block size of 65536 bytes, a buffer size of 262144 cannot be used

Unfortunately, I cannot tweak the settings on the SCSI HBA itself as the Adaptec 29320lpe doesn't have the ability to do this.

Thanks.

(NBU 7.5.0.4 & Win2k8 R2 SP2)

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Mark_Solutions's picture

I have not heard of SAS ignoring buffer sizes - this is something that NBU passes to the device

If the LTO3 SCSI HBA cannot use a larger than 64k block size then the SAS ones would have to use the same size

Having said that - this would apply to old tapes only as once written to the block size is part of the header information and so if you put in a used LTO4 tape with a 256k block size it will write to it using that size

So you could change the setting when you add new tapes and then label them and change it back - not too bad as the change doesnt need a service re-start?

 

To prove it all to yourself  tape a look at the bptm logs file with and without the databuffer setting and see what sioze it says it is using - then run an All Log entries report which will show the throughput obtained for each fragment to see if it has any effect or not

Hope this helps

Authorised Symantec Consultant

Don't forget to "Mark as Solution" if someones advice has solved your issue - and please bring back the Thumbs Up!!.

Richard J Cunningham's picture

Ahh, I remember what my consultant said now - basically, SAS doesn't care what the block size is, but SCSI does.

So if I were to drop the block size down to 64k, but not put any new LTO5's in - it would allow me to write to the LTO3's in 64k, and the LTO5's would carry on writing to their 256k ones, in 256k?

Mark_Solutions's picture

It should do - again you could test this by changing it and running a backup to a used LTO5 and check the bptm log to see what block size it uses - as it doesn't overwrite the header it should use the original block size - it certainly does that for restores when it sets the block size for read operations

Let me know what you find

Authorised Symantec Consultant

Don't forget to "Mark as Solution" if someones advice has solved your issue - and please bring back the Thumbs Up!!.

Yasuhisa Ishikawa's picture

SIZE_DATA_BUFFERS affect all tape backups on this media server. You can not set different buffer size to LTO3 and LTO5.
I prefer to use default buffer size so as to avoid troubles caused by large buffer size setting. Instead, to increase performance, I used to set NUMBER_DATA_BUFFERS higher than default. You may get around 180MB/s(or more) throughput with 64 buffers on standard x86 servers - for more performance, set 128 or 256.

Authorized Symantec Consultant(ASC) Data Protection in Tokyo, Japan

Marianne's picture

You may be able to update registry for your HBA as per this post: https://www-secure.symantec.com/connect/forums/netbackup-70-standalone-drive#comment-4235871 
or NOT, as per this post: https://www-secure.symantec.com/connect/forums/tape-device-index-1-has-maximum-block-size-65536-bytes-buffer-size-262144-cannot-be-used#comment-8115191

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