Re: [RFC] introduce sys_syncat to sync a single file system

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On Thu, 3 Mar 2011 01:22:24 -0600, Jonathan Nieder <jrnieder@xxxxxxxxx> wrote:
> 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.

This would be useful for 9p server in qemu

http://article.gmane.org/gmane.comp.emulators.qemu/95497

> 
> 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).


With some of the proposed changes for VFS [1] some of the *at calls also allows to
specify "" names. So i guess having syncat is useful because now we can
call sync with either an fd or with a name. 

ie 

syncat(fd, "");
or
syncat(AT_FDCWD, "a");

[1] http://article.gmane.org/gmane.linux.file-systems/50773

> 
> 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.
> 

-aneesh
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