linux-kernel - Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK

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Message-ID: <4DB21037.2060604@oracle.com>
Date:	Fri, 22 Apr 2011 16:33:11 -0700
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	Eric Blake <eblake@...hat.com>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...icpanda.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

On 04/22/2011 11:06 AM, Andreas Dilger wrote:
> Sure, there are lots of wasted syscalls, but in this case the cost of doing extra SEEK_DATA and SEEK_HOLE operations may be fairly costly. This involves scanning the whole disk layout, possibly over a network, which might need tens or hundreds of disk seeks to read the metadata, unlike regular SEEK_SET.
>
> Even SEEK_END is somewhat costly on a network filesystem, since the syscall needs to lock the file size in order to determine the end of the file, which can block other threads from writing to the file.
>
> So while I agree that the Linux mantra that "syscalls are cheap" is true if those syscalls don't actually do any work, I think it also ignores the cost of what some syscalls need to do behind the scenes.

You have a point. I was just reviewing the possible patch for ocfs2
and it looks heavy.

One option is to scrap SEEK_* and make another syscall llfind() with FIND_*.
Or, do both.
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