Comment 10 for bug 947664

On Wed, 30 May 2012, Brian J. Murrell wrote:
> Over the same NFS with --force-unsafe-io it takes 26 minutes and that's
> probably because in analysing with strace, I don't see dpkg doing
> anything different per file being installed:

That's because the fsync() are done in a batch at the end of the
unpacking. So the per-file operations at the start are the same...
but when you use --force-unsafe-io you don't have the serie of fsync()
at the end.

> read(9, "/*\n * Generic GPIO API implement"..., 1196) = 1196
> write(10, "/*\n * Generic GPIO API implement"..., 1196) = 1196
> read(9, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 340) = 340
> sync_file_range(0xa, 0, 0, 0) = 0

> So it seems there is still a sync being done after each file is written,
> even with --force-unsafe-io.

sync_file_range() is something different, it just tells the filesystem to
start the writeback, it doesn't wait for it to have happened (contrary to
fsync()).

So it should be irrelevant for the time lost, unless Linux's NFS driver
has a poor handling of this Linux-specific function.

Cheers,
--
Raphaël Hertzog ◈ Debian Developer

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