Thank you to both of you. Things are clearer. And since it's a spare disk and I want to test for example between eSata and USB connections, I will test various things.
And if I find something of value (would it be only the differences of throughput between USB and eSata or the differences of the OS load while reading [resp. writing] in the two cases to the same disk), I would report to the list. Best regards, T. Laronde On Fri, Jul 26, 2019 at 03:23:22PM +0200, Johnny Billquist wrote: > Hi. > > On 2019-07-26 13:41, tlaro...@polynum.com wrote: > >Hello, > > > >On Fri, Jul 26, 2019 at 01:23:34PM +0200, Johnny Billquist wrote: > >>I think it's a serious mistake to use the physical action inside the disk as > >>any kind of indication that there is any actual work going on, from the > >>perspective of system data transfers. > >> > >>The disk can internally be doing various stuff at any point, which is > >>totally irrelevant from this perspective. And disks usually also have the > >>capacity to complete whatever operation is in progress and then move the > >>heads to a safe area in case of power loss. > >> > >>So stop thinking that there is data being written out at a later point after > >>the disk have been unmounted. If the disk is internally caching things, it > >>is still safe to disconnect it. All transfer of data from the OS to the disk > >>have been completed. The disk is free to handle this any way it want > >>internally. But it has to ensure that all data are retained. If it didn't, > >>then essentially it would be useless, as you could not even shut down the > >>system. > >> > > > >I'm not an english native speaker so just to be sure: do you mean it is > >safe to unplug the board/device connection (this is not what is > >bothering me) or do you mean it is safe to cut the power on the disk > >(this is bothering me: if there is activity on the disk, since I can't > >know what exactly it is doing, it must not be put out of power). > > Don't worry about language. Keep asking if anything is unclear. I'm not a > native English speaker either. > > Yes, I do mean it is safe. As soon as umount have returned, the retention of > data is the disks responsibility. If the disk can drop data it has been > entrusted to keep, then the disk is broken. There is no way you can protect > yourself against a broken device, so let's ignore that possibility for now. > If the disk is not broken, then it will retain data that has been entrusted > to it. > > >That the OS has done its work and transfer all the data was my > >assumption. > > Right. So then there is nothing more the OS can do. It's all in the hands of > the disk. You have to assume that the manufacturer of the disk have made > sure that data that has been written to the disk are retained by the disk. > > >My question was: does it mean that the OS can ensure that, when > >the umount has been done, all the data it has transfered has been > >written finally to the disk (physically) or could the disk still > >have the data in cache (for me: memory)---and this is how I interpret > >what you write: as far as the system is concerned, the data has been > >transferred; what the device does with it is another problem, meaning > >that a loss of power will be a loss of data erasing a cache not written > >to disk. > > You don't know, and you should not care. > What you do know is that the disk have received the data, and the disk > guarantees that the data is not lost. > If the disk have a cache with a 100 year battery backup, or if the data was > committed to the spinning rust is irrelevant. If you unplug the disk, and > later plug it in again, the data that you wrote will still be there, and > will be read out the same way you wrote it. > > The disk internally is actually free to do all kind of things, as long as it > fulfills the implicit and explicit requirements and expectations of the > protocol between the controller and disk. > Among those are that if data have been written to the disk, and the disk > have acknowledged this, then the data will be there, even if power is lost. > > It is not acceptable for the disk to acknowledge data as written, and loose > it if power is lost after that. > > Down that path lies madness. Because then you would never be able to trust > the disk, and you would never be able to even shut the system down and power > it off in an orderly fashion. > > >Specially: if mounting with "sync" does the OS has the mean to > >ensure that the data is written physically to the disk (that could > >be powered off), waiting to return from umount until it is actually > >done---while without sync, it can return once the data is transferred > >as far as it (the OS) is concerned, but not waiting for the device > >to tell it has been "committed" actually to disk. > > No. The OS have no way of fully control what a disk does internally. And it > wouldn't even make sense to try and have that kind of control. > > But you can be sure that the data you wrote will be possible to read back, > even if you have a power loss immediately after the write, no matter what > kind of caches and optimizations the disk do internally. > > Johnny > > -- > Johnny Billquist || "I'm on a bus > || on a psychedelic trip > email: b...@softjar.se || Reading murder books > pdp is alive! || tryin' to stay hip" - B. Idol -- Thierry Laronde <tlaronde +AT+ polynum +dot+ com> http://www.kergis.com/ http://www.sbfa.fr/ Key fingerprint = 0FF7 E906 FBAF FE95 FD89 250D 52B1 AE95 6006 F40C