:: Re: [DNG] noatime by default
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著者: Didier Kryn
日付:  
To: dng
題目: Re: [DNG] noatime by default
Le 24/08/2017 à 09:46, Narcis Garcia a écrit :
> El 24/08/17 a les 01:03, Adam Borowski ha escrit:
>> Hi!
>> I'd like to recommend another improvement: let's make the installer default
>> to noatime for fstab it creates.
>>
>> In the past, atime updates used to ruin performance.  Thanks to work by Ted
>> Ts'o and others, that penalty has been greatly reduced (but not eliminated)
>> by two options:
>> * relatime (on by default): atime is not updated unless atime<=mtime or
>>    atime<NOW-24h
>> * lazytime (off by default): atime and mtime updates are postponed by up to
>>    30 minutes, unless there's memory pressure or the inode has to be written
>>    for another reason

>>
>> But alas, there are more considerations than just performance:
>> * atime is really nasty for any CoW filesystems (btrfs, zfs, lvm snapshots,
>>    qcow2 snapshots, deduped thin storage, etc).  For a /usr-y mix of files,
>>    it costs around 5% of disk space per snapshot.  relatime offers no help as
>>    its threshold (24h) just happens to match the most popular snapshot and
>>    cronjob frequency (once per day).  Even worse, it can make _reading_
>>    things result in ENOSPC!
>> * atime murders media with sharply limited write endurance, such as SD
>>    cards.  For this reason, every SoC pre-made image I've seen ships with
>>    noatime.

>>
>> And what do we get in return?
>> * the "vote" field in popcon.  This somewhat benefits the distribution (lets
>>    us spot low-use packages installed by default, such as xterm (53.60% inst,
>>    6.67% vote) or tcpd (95.20% inst, 9.85% vote)) but gives no gain to
>>    individual users.
>> * some rare forensic uses
>> * new mail notification on login

>>
>> The last thing has recentlish (POSIX-2008) gained a solution: mail readers
>> can be patched to manually calls futimens(f, {UTIME_NOW, UTIME_OMIT});
>> stretch/ascii already sports mutt patched this way -- not sure what other
>> mail clients are likely to be used by local users thus worthy of patching.
>> Sample patch:
>> https://github.com/neomutt/neomutt/commit/816095bfdb72caafd8845e8fb28cbc8c6afc114f
>> Because of stretch's freeze, I did not manage to write+push upstream patches
>> for any client other than mutt, I guess it's time to resume.
>>
>> Obviously, an admin who thinks he actually has an use for atime is free to
>> edit fstab.
>>
>> So, what would you folks say about defaulting to noatime?
>>
>>
>> Meow!
>>
> Since 10 years ago I'm setting "noatime" to all mounts in thousands of
> GNU/Linux installations.
> Yes, please set "noatime" as default for any volume.


     +1
     When I first formatted all partitions with Reiserfs, which was the 
first journaled filesystem, it was recommended to set noatime for 
performance. There was no visible drawback. With SSDs, atime is mostly 
harmfull. I would recommend also nodiratime, which is not implied by 
noatime. And I think that none of these are implied by mounting rdonly.


     Didier