# Re: \[RFC\] introduce sys\_syncat to sync a single file system

- List: linux-api
- 2 messages, 2 authors
- Page: https://ratatoskr.run/linux-api/2011/03/51341/t

## Re: \[RFC\] introduce sys\_syncat to sync a single file system

- From: Jonathan Nieder \<hidden\>
- Date: 2011-03-03 07:22:24 
- Message: https://ratatoskr.run/linux-api/2011/03/51341.md

```text
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
```

## Re: \[RFC\] introduce sys\_syncat to sync a single file system

- From: Aneesh Kumar K. V \<hidden\>
- Date: 2011-03-03 08:54:56 
- Message: https://ratatoskr.run/linux-api/2011/03/51378.md

```text
On Thu, 3 Mar 2011 01:22:24 -0600, Jonathan Nieder [ref] 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
```