linux-kernel - Re: [rfc] new stat*fs-like syscall?

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Date:	Thu, 24 Jun 2010 16:03:05 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Nick Piggin <npiggin@...e.de>
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	viro@...IV.linux.org.uk, drepper@...hat.com,
	torvalds@...ux-foundation.org
Subject: Re: [rfc] new stat*fs-like syscall?

On Thu, 24 Jun 2010, Nick Piggin wrote:
> This has come up a few times in the past, and I'd like to try to get
> an agreement on it. statvfs(2) importantly contains f_flag (mount
> flags), and is encouraged to use rather than statfs(2). The kernel
> provides a statfs syscall only.
> 
> This means glibc has to provide f_flag support by parsing /proc/mounts
> and stat(2)ing mount points. This is really slow, and /proc/mounts is
> hard for the kernel to provide. It's actually the last scalability
> bottleneck in the core vfs for dbench (samba) after my patches.
> 
> Not only that, but it's racy.
> 
> Other than types, other differences are:
> - statvfs(2) has is f_frsize, which seems fairly useless.

statfs(2) also has f_frsize since 2.6.0, only it hasn't been
documented (should be fixed now).

> - statvfs(2) has f_favail.
> - statfs(2) f_bsize is optimal transfer block, statvfs(2) f_bsize is fs
>   block size. The latter could be useful for disk space algorithms.
>   Both can be ill defned.

They are the same, only the documentation is different.

> - statvfs(2) lacks f_type.
> 
> Is there anything more we should add here? Samba wants a capabilities
> field, with things like sparse files, quotas, compression, encryption,
> case preserving/sensitive.
> 
> Any thoughts?

"struct statfs" and "struct statfs64" have spare fields.  We could put
the f_flag in there including a magic "this is a valid f_flag" flag,
that distinguishes from the default zero value.

Thanks,
Miklos
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