linux-kernel - Re: [PATCH v3] introduce sys_syncfs to sync a single file system

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Date:	Sat, 12 Mar 2011 00:45:47 +0100 (CET)
From:	"Indan Zupancic" <indan@....nu>
To:	"Arnd Bergmann" <arnd@...db.de>
Cc:	"Sage Weil" <sage@...dream.net>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Jonathan Nieder" <jrnieder@...il.com>, akpm@...ux-foundation.org,
	linux-api@...r.kernel.org, mtk.manpages@...il.com,
	viro@...iv.linux.org.uk, hch@....de, l@...per.es
Subject: Re: [PATCH v3] introduce sys_syncfs to sync a single file system

On Fri, March 11, 2011 12:55, Arnd Bergmann wrote:
> On Friday 11 March 2011, Indan Zupancic wrote:
>> >       http://marc.info/?l=linux-fsdevel&amp;m=127970513829285&amp;w=2
>>
>> The patch there seems much more reasonable than introducing a whole
>> new systemcall just for 20 lines of kernel code. New system calls are
>> added too easily nowadays.
>
> The only problem with adding new system calls is that we are stuck
> with the interface until the end of time, so we must be sure not
> to get it wrong. The same thing is true for any other interface
> such as ioctl or extensions to existing system calls. People usually
> get away with adding new ioctls more easily because it is less
> obvious when they are added.

Agreed.

I'm not sure this feature is important enough to add. I can't really
think of a regular use case where this would be useful, generally
it's transparent on which mount files are. Add symlinks, and you
give users a lot of rope. Any user has to make sure that all the
files they want to sync are on the same file system.

About the arguments against sync(2):

>  - On machines with many 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., /).

It would be better to fix NFS, or mount it with the fsc option (assuming
a sync will write to the local cache instead of hanging forever then).

>  - Some applications 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.

You can use sync_file_range() on those files to schedule the writes
and then do the fsync(2) as usual (both on files and dirs).

If there still is a good reason to implement this, please don't add it
as a new system call, but add it to sync_file_range(), as that seems
the best place for odd file synchronisation operations.

Greetings,

Indan


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