From: Eric Sandeen <sandeen@sandeen.net>
To: Andy Lutomirski <luto@amacapital.net>,
Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>, Sage Weil <sweil@redhat.com>,
Linux API <linux-api@vger.kernel.org>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Zach Brown <zab@redhat.com>
Subject: Re: [PATCH RFC] vfs: add a O_NOMTIME flag
Date: Fri, 08 May 2015 09:44:25 -0500 [thread overview]
Message-ID: <554CCBC9.3070706@sandeen.net> (raw)
In-Reply-To: <CALCETrUksu5ZB4QBfC8DMwYO2OFjfPW2eWsTweZGN_gybzcsmw@mail.gmail.com>
On 5/7/15 10:24 PM, Andy Lutomirski wrote:
> On May 8, 2015 8:11 AM, "Dave Chinner" <david@fromorbit.com> wrote:
>>
>> On Thu, May 07, 2015 at 10:20:53AM -0700, Zach Brown wrote:
>>> On Thu, May 07, 2015 at 10:26:17AM +1000, Dave Chinner wrote:
>>>> On Wed, May 06, 2015 at 03:00:12PM -0700, Zach Brown wrote:
>>>>> Add the O_NOMTIME flag which prevents mtime from being updated which can
>>>>> greatly reduce the IO overhead of writes to allocated and initialized
>>>>> regions of files.
>>>>
>>>> Hmmm. How do backup programs now work out if the file has changed
>>>> and hence needs copying again? ie. applications using this will
>>>> break other critical infrastructure in subtle ways.
>>>
>>> By using backup infrastructure that doesn't use cmtime. Like btrfs
>>> send/recv. Or application level backups that know how to do
>>> incrementals from metadata in giant database files, say, without
>>> walking, comparing, and copying the entire thing.
>>
>> "Use magical thing that doesn't exist"? Really?
>>
>> e.g. you can't do incremental backups with tools like xfsdump if
>> mtime is not being updated. The last thing an admin wants when
>> doing disaster recovery is to find out that the app started using
>> O_NOMTIME as a result of the upgrade they did 6 months ago. Hence
>> the last 6 months of production data isn't in the backups despite
>> the backup procedure having been extensively tested and verified
>> when it was first put in place.
>>
>>>>> The criteria for using O_NOMTIME is the same as for using O_NOATIME:
>>>>> owning the file or having the CAP_FOWNER capability. If we're not
>>>>> comfortable allowing owners to prevent mtime/ctime updates then we
>>>>> should add a tunable to allow O_NOMTIME. Maybe a mount option?
>>>>
>>>> I dislike "turn off safety for performance" options because Joe
>>>> SpeedRacer will always select performance over safety.
>>>
>>> Well, for ceph there's no safety concern. They never use cmtime in
>>> these files.
>>
>> Understood.
>>
>>> So are you suggesting not implementing this
>>
>> No.
>>
>>> Or are we talking about adding some speed bumps
>>> that ceph can flip on that might give Joe Speedracer pause?
>>
>> Yes, but not just Joe Speedracer - if it can be turned on silently
>> by apps then it's a great big landmine that most users and sysadmins
>> will not know about until it is too late.
>
> What about programs like tar that explicitly override mtime? No admin
> buy-in is required for that. Admittedly, that doesn't affect ctime,
> nor is it as likely to bite unexpectedly as a nomtime flag.
>
> I think it would be reasonably safe if a mount option had to be set to
> allow O_NOCMTIME or such.
I was going to suggest the same. Make infrastructure available for an app
to request O_NOMTIME, but a mount option must be set to allow it, so the
administrator doesn't get an unhappy surprise at backup-restore time.
(Not a big fan of more twiddly knobs, but that seems to put the control
in all the right places).
-Eric
next prev parent reply other threads:[~2015-05-08 14:44 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-06 22:00 [PATCH RFC] vfs: add a O_NOMTIME flag Zach Brown
2015-05-06 22:14 ` Trond Myklebust
2015-05-06 22:19 ` Sage Weil
2015-05-06 22:41 ` Zach Brown
2015-05-06 22:46 ` Sage Weil
2015-05-06 23:21 ` Theodore Ts'o
2015-05-07 0:26 ` Dave Chinner
2015-05-07 17:20 ` Zach Brown
2015-05-07 18:43 ` Zach Brown
2015-05-07 19:09 ` Richard Weinberger
2015-05-07 19:53 ` Andy Lutomirski
[not found] ` <554BC4D8.9010507@nod.at>
2015-05-07 20:06 ` Andy Lutomirski
2015-05-08 2:42 ` Dave Chinner
2015-07-14 11:44 ` Pavel Machek
2015-05-08 1:01 ` Sage Weil
2015-05-08 1:23 ` Trond Myklebust
2015-05-08 15:19 ` Sage Weil
2015-05-08 22:13 ` Dave Chinner
2015-05-08 22:24 ` Sage Weil
2015-05-10 23:13 ` Trond Myklebust
2015-05-11 7:31 ` Dave Chinner
2015-05-11 16:39 ` Sage Weil
2015-05-11 17:12 ` Trond Myklebust
2015-05-11 17:30 ` Sage Weil
2015-05-12 1:21 ` Dave Chinner
2015-05-12 23:12 ` Sage Weil
2015-05-13 0:57 ` Dave Chinner
2015-05-12 13:41 ` John Stoffel
2015-05-11 14:47 ` Theodore Ts'o
2015-05-11 16:24 ` Sage Weil
2015-05-11 23:10 ` Theodore Ts'o
2015-05-12 5:08 ` Kevin Easton
2015-05-12 11:45 ` Austin S Hemmelgarn
2015-05-12 13:54 ` John Stoffel
2015-05-12 14:36 ` J. Bruce Fields
2015-05-12 14:53 ` Austin S Hemmelgarn
2015-05-12 21:51 ` Dave Chinner
2015-05-13 15:16 ` Austin S Hemmelgarn
2015-05-12 22:39 ` NeilBrown
2015-07-14 13:13 ` Pavel Machek
2015-07-15 4:54 ` NeilBrown
2015-07-22 13:47 ` Pavel Machek
2015-05-12 21:35 ` Sage Weil
2015-05-13 12:32 ` Jan Kara
2015-05-08 14:29 ` John Stoffel
2015-07-14 11:50 ` Pavel Machek
2015-05-08 14:43 ` Austin S Hemmelgarn
2015-05-08 17:11 ` Zach Brown
2015-05-08 2:37 ` Dave Chinner
2015-05-08 3:24 ` Andy Lutomirski
2015-05-08 14:44 ` Eric Sandeen [this message]
2015-05-11 20:36 ` J. Bruce Fields
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