Tar to tape : block number and blocking factor
The 64 kiB blocksize is to maximize throughput and avoid "shoe shining".
mt -f /dev/nst0 setblk 64k
tar -c -v -R -b128 -f /dev/nst0 test_dir
returns:
bloc 0 : test_dir/
bloc 1 : test_dir/file_1.bin
bloc 204802 : test_dir/file_2.bin
bloc 2252803 : test_dir/file_3.bin
bloc 4300804 : test_dir/file_4.bin
...
But the block number in the tar output corresponds in 512 B blocksize, despite blocking factor giving 64 kiB blocksize (128 * 512).
And then, regardless of blocksize of the mt command (variable, 64 kiB).
The goal would to random access in the Tar-tape. Convert 64 kiB blocksize to 512 implies to trim.
Is there a way to matching records size of tar and mt ?
UPDATE: Arg! So much for tests. Just did a full backup and tar with its default block/record size ran about 25% slower. So I'm back to using -b2048. Will try -b1024 and see how it goes.
Okay, just ran some tests. My advice is to set the tape block size to 0 (variable) and choose a tar block/record of your choice. I tested using a tape block size of 1M and 512K with matching tar record sizes (-b2048=1M -b1024=512K) and then I set the tape block size to 0 and tested using tar -b2048 and -b1024 and there was no difference. Then I ran a test with 'setblk 0' again but with a default tar block size of 512 (no -bxxxx in other words) for total record length of 10240 bytes and still no difference in performance.
I'm using a Quantum LTO-5. As long as you're substantially above 512 bytes (the LTO-5 default, I believe) you should be okay and it's unlikely you will experience any shoe-shining. IMO, the only reason to set the drive block size (instead of variable) is when its ignoring the software block size (record size in case of tar).
Note: the default block size for tar is 512 bytes x 20 for a total "record size" of 10240 bytes. Btw, my tests all finished within 12 seconds which totals ~141,000,000 bytes / second, the LTO-5 max throughput.
My results with --blocking-factor using tar to tape on multiple 200 to 300 GB size tar archives.
--blocking-factor 256 throughput 62 to 68 MiB/sec.
--blocking-factor 1024 throughput 87 MiB/sec.
I intend to do more experimentation with even larger blocking factors.
Above obtained with default hardware block size (I think variable) and no compression.
My equipment listed below:
HP EH958B StorageWorks Ultrium 3000 LTO-5 1.5/3TB SAS (Serial Attached SCSI) Half-Height External Tape Drive LTO5
TDK LTO-5 Ultrium Data Cartridge 1.5 TB / 3.0 TB LTO Ultrium-5 Tape
ATTO Technology ExpressSAS H680 PCIe 2.0 Low Profile 6Gb/s SAS HBA Card (External Ports) P/N: ESAS-H680-000