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