Hi,
Sage Weil wrote:
> - On machines with many of mounts, it is not at all uncommon for some of
> them to hang (e.g. unresponsive NFS server). sync(2) will get stuck on
> those and may never get to the one you do care about (e.g., /).
Fun to see this again.
> - Some applications (Ceph, dpkg) write lots of data to the file system and
> then want to make sure it is flushed to disk. Calling fsync(2) on each
> file introduces unnecessary ordering constraints that result in a large
> amount of sub-optimal writeback/flush/commit behavior by the file
> system.
FWIW dpkg uses sync_file_range(2) and only syncs the files it needs to
nowadays. Other apps in the same position should probably do the
same.[1][2]
> This patch introduces a new system call syncat(2) that mimics the existing
> *at() interfaces by taking an fd and/or path. The fd can be either an
> open file descriptor or AT_FDCWD, and the pathname can be either a path or
> (unlike the usual *at() style interface) NULL. Only the file system for
> the referenced file is synced.
Sounds like overengineering. The openat(2) family of calls are meant
to add flexibility to familiar calls that perform an operation with a
path relative to the cwd. To maintain familiarity, they include some
complication (AT_FDCWD, taking a relative path, and so on).
Since sync_one_filesystem(2) is new, why not just take a file or
directory fd (and perhaps flags for future expansion)? I can use
open(".", O_NONBLOCK) to get a file descriptor for the cwd.
> Is this a reasonable approach? (Patch below is compile tested only. :)
Sounds reasonably sane.
As for the patch: without the pathname arg it becomes much simpler.
To my inexpert eyes, aside from that it looks good.
Thanks,
Jonathan
[1] http://thread.gmane.org/gmane.comp.file-systems.ext4/22190
[2] http://lists.debian.org/debian-dpkg/2010/11/threads.html#00075
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