On Fri, Sep 13, 2024 at 8:00 AM Anssi Saari
<anssi.sa...@debian-user.mail.kapsi.fi> wrote:
>
> David Wright <deb...@lionunicorn.co.uk> writes:
>
> > On Tue 10 Sep 2024 at 11:56:25 (+0300), Anssi Saari wrote:
>
> >> Why do you think that? Which part of the fsync manpage explicitly covers
> >> fsync's effect on device files? Share share, it's fair.
>
> > “fsync() transfers ("flushes") all modified in-core data of (i.e.,
> > modified buffer cache pages for) the file referred to by the file
> > descriptor fd to the disk device (or other permanent storage device)
> > so that all changed information can be retrieved even after the
> > system crashed or was rebooted. This includes writing through or
> > flushing a disk cache if present. The call blocks until the device
> > reports that the transfer has completed. It also flushes metadata
> > information associated with the file (see stat(2)).”
> You just parroted a man page I had already read. Why did you think
> that'd be helpful? I asked the questions because the man page didn't
> answer my questions. As you are apparemntly unable I found for example
> https://unix.stackexchange.com/questions/473854/block-device-cache-v-s-a-filesystem
> and so arrived to the conclusion that the final close(2) call on a block
> device already flushes all buffers before returning. So the answer to
> the question "is running sync needed after dd to block device" is
> no. Someone else posted that too on this list recently, in another
> thread.
To add a datapoint...
My daily driver workstation is really fast with lots of RAM. It has
3.4 GHz cpu and 64 GB of RAM. I also set swappiness to a low value to
avoid spilling out of RAM.
I use a lot of SBC's/dev boards for testing. They usually use a
SDcard. The SDcard is really slow. The card can only provide 10 MB/s
or 30 MB/s write speeds. Some cards I use are so cheap they are even
slower.
If I dd an image from the workstation to the SDcard, it happens in
under a second. dd exits, and closes its file descriptors. Something
is obviously wrong since the image is 1 or 2 GB, and the SDcard write
speed is 10 MB/s or 30 MB/s.
The file system cache is still holding the writes. If I remove the
SDcard and try to use it, the image is corrupt. When I say "remove", I
mean pop the card out of the card reader since the write has
supposedly finished.
What I found is, I have to manually call sync to ensure the image is
written from cache to the SDcard. When I call `dd if=... of=/dev/sdd
&& sync`, the command takes 30 seconds or so to complete. The time is
spent in sync, not dd.
Based on my experience with lots of RAM and slow media, you have to
call sync to get the cache manager to write back to the disk.
Jeff
