Using StorageWorks 24 USB with Linux

[Samy Touati]

Hi,

I connected a StorageWorks 24 USB to Fedora core 4 using a 2.6.16 kernel with success: the drives is recognized and accessible for read/write.
I'm trying to figure out the most optimum block size value for this drive.
When I type mt -f /dev/st0 stat, it shows a block size of 0 which is variable.

Should I leave it as is (variable), or is there a value that would be optimal for this drive.
Tar and cpio have block size parameters as well, are they connected to the block size that can be set using the mt command for the drive?

I read somewhere that DDS tapes/drives don't use separators as QIC tapes do, so setting block size in tar or using mt is irrelevant for them.

So far I'm able to write 10G of data using no compression in 57 minutes with a block size value of 240 using tar with the drive set to variable block size.

Thanks.

Samy.

[DanC_1]

Hi Samy,

Leaving the drive set in variable mode is an optimum thing to do. (it will revert to that on reboot anyway).
Yes it is worth playing with the blocksize parameters in tar or cpio. Set the transfers to around 64KB or higher for best performance. (remember in tar that blocksize is in 512byte increments so tar cbf 128 ... will get you 64KB)
DDS drives can use filemarks and setmarks, but tar won't use them.
You can test your performance using a tool provided by HP called hptapeperf (search for it on the support site), this writes data to tape from memory so provides you with a maximum performance figure for your system.
For a more fully featured backup tool why not try the software provided in the box with you drive, this supports linux.

[Samy Touati]

I played with the block size factor of tar and didn't notice any difference in speed.
I tried the default of 20 (20 * 512bytes) up until 240 (240 * 512bytes).
I noticed that I cannot go beyond 240.
When using a block size of 240 with tar I cannot do multi-volume: the linux machine hangs/crash after 30 seconds.
If I use the regular block size of 20 * 512 then I can do multi-volume backup with tar.

My unit got bundled with tapeware which doesn't support storageworks with usb connections, and I cannot get HP Data Protector Express single server edition which does support linux/usb drives.

Should the block size of tar affect speed or affect volume of data on the tape itself?


Samy.

[Samy Touati]

Hi again,

Just some clarifications on my previous posting.

The software bundled with my drive is tapwware which is not compatible with USB drives on Linux. Unfortunately there is no way to get the single server edition (even paying for it).

I prefer using tar/cpio as they are will be still available in 5 years from now if I want to restore some data.

In playing with the different block sizes of tar I didn't notice any change in speed for writing/reading.
I was able to set a block size of 240 for tar (240 * 512). If I put a larger value I get an error message telling that my block size is not a multiple of the block size of the tape.
Does this mean that this is the max supported?
Is it safe for compatibility to go that far in block sizes.

Thanks again for the help.

Samy.

[DanC_1 · Accepted solution]

Hi Samy,

I actually think that the reason you aren't seeing any performance improvement is that you are already maxing out the drive! DAT24 isn't a high performance device, my estimate (based on the numbers you gave) is that you are getting more or less 3MB/s which is pretty much the max for the device with data of normal compressibility.
DAT40 and DAT72 devices will do 3MB/s native (6MBs/s compressed).

The crash problems that you are seeing with the large blocksizes are probably due to maximum driver transfer sizes in Linux, I wouldn't worry too much about going up to blocksizes that large as there will be negligable (or no) effect of performance above 64KB. So best stick to smaller values. A tar blocksize value of 128 is more than sufficient.

[Samy Touati]

Hello,

Thanks so much for the explanation.
I'll then stick with the 64K for tar/cpio as reccommended.

Samy.