What is blocksize?

binarybob0001 · 04-20-2006, 10:26 PM

Pretty simple question. I see people using this with dd all the time. What difference does it make when you use a block size of say 1M in comparison to 512? Thanks.

jonaskoelker · 04-21-2006, 01:14 AM

Quote:

Originally Posted by binarybob0001

What difference does it make when you use a block size of say 1M in comparison to 512? Thanks.

Obviously dd will copy the data in larger chunks per read and write operation. This means fewer calls to read(2) and write(2), so fewer context switches. If this is the only effect, you should expect a (very slight) speedup.

However, there is probably some gain I don't know about by copying in block sizes that match those of the hardware and/or file system.

Finally, it may be affected by local variance, cosmic rays and ISO standards. So, go measure it for yourself.

hth --Jonas

debasish_5849 · 04-21-2006, 07:06 AM

not really related to the forum topic jonaskoelker. But would you please tell me where you found the Microsofts guilty things you mentioned in your signature?

binarybob0001 · 04-21-2006, 03:29 PM

OK, I will test them, but first I need to figure out how to find the blocksize of my hardware. I will need to know the blocksize of my hardware though. How do I find that out?

jonaskoelker · 04-21-2006, 06:29 PM

Quote:

Originally Posted by binarybob0001

I will need to know the blocksize of my hardware though. How do I find that out?

Don't know, but 512 bytes is a pretty sound guess.

OT: I can't recall where I find out that microsoft was indeed found guilty of anti-trust violations, but I'm sure that your favorite search engine is a good place to start.

pixellany · 04-21-2006, 06:59 PM

Quote:

Originally Posted by binarybob0001

OK, I will test them, but first I need to figure out how to find the blocksize of my hardware. I will need to know the blocksize of my hardware though. How do I find that out?

I don't think blocksize is a property of hardware. For example, if you are using dd to put thing on a hard disk, you can use just about any block size you want. Probably the same answer for a floppy or a usb memory stick.

Obviously, using a block size larger than the capacity of the target device would not make sense......

jonaskoelker · 04-21-2006, 07:27 PM

pixellany: you're not wrong, but you're not completely right either. Basically, the word `block' menas something to dd, and something different to the disk, and something completely third to the file system.

As I alluded to, making the size of a dd block be the size of a disk block and/or fs block may have advantages.