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NBU-Duplication issue

hi guys,

 

i'm facing a strange issue when duplication images from my AdvDisk...

if i run backup with my Netbackup 7.5.0.5 to Advdisk its successful and duplicating simultaniously to puredisk the performance is  OK up to  (87000KB/SEC)

if i run backup with my Netbackup 7.5.0.5 to Advdisk its successful and duplicating simultaniously to tape LTO4 the performance is ok  OK up to 70000KB/SEC)

BUT

if duplication to both PDDO and LTO4 in the same time the performance is 43 000 KB/SEC

i spent many days reading the logs ( nbsl, nbpem, nbjm nbrb , nbstserv ,vxlogcfg etc ) i couldnt find any errors or a source limiting the performance

all what seems to me strange is when writing to tape the bptm  reporting in more than 300 lines :

write_data: writing block shorter than BUFF_SIZE, 65536 bytes

write_data: writing short block, 65536 bytes, remainder 0

seems it's related to what previously  found :

DUP_BLKSIZE  ( in bptm) are by default set to 65536 (64K) so when we duplicate to tape the read requests  pass the data in 64 , and the backups written to Advanced disk  normally on the disk array the data is being written  in 512K if it’s not  more, if  we will duplicate by passing data in 64K which is insufficient  because it will require more (Input/Output Per Second) = the performance decreasing …..

 

IS it related ????

the Master is an AIX, Appliances 5220

any help sugesstion ?

thank you in advance