SIZE_DATA_BUFFERS... Mix of SAS&SCSI.

[Richard_J_Cunni]

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)

[Mark_Solutions]

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

[Richard_J_Cunni]

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]

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

[Yasuhisa_Ishika]

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.

[Marianne]

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-byte...