On Sat, Dec 19, 2009 at 10:15 AM, Neil Bothwick <n...@digimed.co.uk> wrote: > On Sat, 19 Dec 2009 12:11:27 +0000, Stroller wrote: > >> > Incidentally, if you want to use dd, adding bs=4096 speeds it up quite >> > significantly. >> >> Thank you. I have always wondered what the optimal bs might be. >> And why - could you possibly explain that, please? >> >> Is bs=4096 best for all disk-based operations? > > Many filesystems are set up with 4K blocks, so matching this with dd is > more efficient. The default is 512 byte blocks and anything larger > than this is good, I sometimes use 40960 but that isn't significantly > faster. I prefer to avoid using dd on hard disks altogether, it's just > so damn slow for large amounts of data. > > > -- > Neil Bothwick > > You can't teach a new mouse old clicks. >
My *completely uneducated* guess would be that, for a raw disk level copy (on a normal spinning disk) or write a bs that is *at the least* divisible into the drive's cache size, and at best *is* the drive's cache size, would be best. For SSDs, if you have some strange reason to need to use dd with one (I'd avoid it simply because a: you'll never guarantee an overwrite of what's really there now and b: you'll be put at least a small dent in the lifespan of the drive) the minimum erase block size would be best, since that'd allow both a full erase and a full write of a block, rather than risking 2 erases to get all of one block written. I do reiterate that this is all mere conjecture, and is based in my likely flawed conceptual understanding of the drives. -- Poison [BLX] Joshua M. Murphy
