linux-ext4 - Re: Documenting MS

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Message-ID: <54F02C73.5090601@gmail.com>
Date:	Fri, 27 Feb 2015 09:36:03 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Omar Sandoval <osandov@...ndov.com>
CC:	mtk.manpages@...il.com, Theodore Ts'o <tytso@....edu>,
	Eric Sandeen <sandeen@...hat.com>, linux-man@...r.kernel.org,
	Linux API <linux-api@...r.kernel.org>,
	XFS Developers <xfs@....sgi.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Linux btrfs Developers List <linux-btrfs@...r.kernel.org>
Subject: Re: Documenting MS_LAZYTIME

Hello Omar,

On 02/27/2015 09:08 AM, Omar Sandoval wrote:
> On Fri, Feb 27, 2015 at 09:01:10AM +0100, Michael Kerrisk (man-pages) wrote:
>> On 02/27/2015 01:04 AM, Theodore Ts'o wrote:
>>> On Thu, Feb 26, 2015 at 02:36:33PM +0100, Michael Kerrisk (man-pages) wrote:
>>>>
>>>>     The disadvantage of MS_STRICTATIME | MS_LAZYTIME is that
>>>>     in the case of a system crash, the atime and mtime fields
>>>>     on disk might be out of date by at most 24 hours.
>>>
>>> I'd change to "The disadvantage of MS_LAZYTIME is that..."  and
>>> perhaps move that so it's clear it applies to any use of MS_LAZYTIME
>>> has this as a downside.
>>>
>>> Does that make sense?
>>
>> Thanks, Ted. Got it. So, now we have:
>>
>>        MS_LAZYTIME (since Linux 3.20)
>>               Reduce  on-disk  updates  of  inode  timestamps  (atime,
>>               mtime, ctime) by maintaining these changes only in  mem‐
>>               ory.  The on-disk timestamps are updated only when:
>>
>>               (a)  the inode needs to be updated for some change unre‐
>>                    lated to file timestamps;
>>
>>               (b)  the application  employs  fsync(2),  syncfs(2),  or
>>                    sync(2);
>>
>>               (c)  an undeleted inode is evicted from memory; or
>>
>>               (d)  more  than 24 hours have passed since the inode was
>>                    written to disk.
>>
>>               This mount significantly reduces writes needed to update
> "This mount option"?

Thanks, fixed.

>>               the  inode's  timestamps,  especially  mtime  and atime.
>>               However, in the event of a system crash, the  atime  and
>>               mtime  fields  on  disk might be out of date by up to 24
>>               hours.
>>
>>               Examples of workloads where this option could be of sig‐
>>               nificant  benefit include frequent random writes to pre‐
>>               allocated files, as well as cases where the  MS_STRICTA‐
>>               TIME  mount  option  is also enabled.  (The advantage of
>>               (MS_STRICTATIME |  MS_LAZYTIME)  is  that  stat(2)  will
>>               return  the  correctly  updated  atime,  but  the  atime
>>               updates will be flushed to disk only when (1) the  inode
>>               needs  to  be  updated for filesystem / data consistency
>>               reasons or (2) the inode is pushed out of memory, or (3)
>>               the filesystem is unmounted.)
> Is it necessary to repeat the reasons for flushing, which are stated
> above?

Good point. I replaced this piece with just a few words referring 
to the list above.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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