On Thu, Sep 05, 2024 at 11:25:33AM +0200, Thomas Schmitt wrote: > Hi, > > to...@tuxteam.de wrote: > > This is exactly why I'm always trying to "sell" the "oflag=sync" option of > > dd. You are going to write each block exactly once, then plop the medium > > out. So whithout, it'll take you 10 sec, with it'll take you as much. But > > it's more transparent with, because without, the OS is flushing cache in > > the background. > > I let xorriso-dd-target use > bs=1M oflag=dsync > with the dd run which copies the ISO image. Not only to minimize the time > of the final sync command but also to get realistic numbers from > status=progress > which elsewise reports fantastic write speeds in the beginning and then > looks quite erratic when the kernel decides to write data to the USB stick > before buffering more of the data sent by dd. > > What i experienced as puzzling in the documentation was the difference > between oflag=dsync and oflag=sync (not to be confused with conv=sync). > The info document of dd is more verbous than the man page but still not > really helpful in this aspect: > > ‘dsync’ > Use synchronized I/O for data [...]
> ‘sync’ > Use synchronized I/O for both data and metadata. [...] > When writing to a device file, neither its length nor timestamp are of > importance for the resulting state of the USB stick [...] That was my take, too: in the "USB stick" case, there's practically no difference. And when writing to a file, I'll happily take the hit of updating the metadata too for consistent results. So I decided to burden my dwindling memory with one letter less :-) > change during writing and the timestamp vanishes with the device file > when the stick gets unplugged. > Nevertheless i add a standalone sync command to the last dd run of > xorriso-dd-target, just to care for any other remaining buffered data > e.g. from zeroizing the potential GPT backup header at the end of the > USB stick. You know far more about those innards than I can hope too :-) > Michael Stone wrote: > > It's a waste of time > > My experience with or without bs=1M oflag=dsync is like the sniffles: > They last seven days with a doctor and a week without. The sticks I've used (and the CPU and what not) are *much* faster with bs=1M than with the default (512, with bs). The oflag=sync is more about predictability, as you write above. Cheers -- t
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