Re: Documenting MS_LAZYTIME

Ted,

On 02/21/2015 03:56 AM, Theodore Ts'o wrote:

On Fri, Feb 20, 2015 at 09:49:34AM -0600, Eric Sandeen wrote:

quoted

quoted

              This mount option significantly reduces  writes  to  the
              inode  table  for workloads that perform frequent random
              writes to preallocated files.

This seems like an overly specific description of a single workload out
of many which may benefit, but what do others think?  "inode table" is also
fairly extN-specific.

How about somethign like "This mount significantly reduces writes
needed to update the inode's timestamps, especially mtime and actime.

What is "actime" in the preceding line? Should it be "ctime"?

Examples of workloads where this could be a large win include frequent
random writes to preallocated files, as well as cases where the
MS_STRICTATIME mount option is enabled."?

I think some version of the following text could also usefully go 
into the page, but...

(The advantage of MS_STRICTATIME | MS_LAZYTIME is that stat system
calls will return the correctly updated atime, but those atime updates
won't get flushed to disk unless the inode needs to be updated for
file system / data consistency reasons, or when the inode is pushed
out of memory, or when the file system is unmounted.)

I find the wording of there a little confusing. Is the following 
a correct rewrite:

    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.)

?

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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On Thu, Feb 26, 2015 at 09:49:39AM +0100, Michael Kerrisk (man-pages) wrote:

quoted

How about somethign like "This mount significantly reduces writes
needed to update the inode's timestamps, especially mtime and actime.

What is "actime" in the preceding line? Should it be "ctime"?

Sorry, no, it should be "atime".

I find the wording of there a little confusing. Is the following 
a correct rewrite:

    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.)

Yes, that's correct.  The only other thing I might add is that in the
case of a crash, the atime (or mtime) fields on disk might be out of
date by at most 24 hours.

						- Ted

On 02/26/2015 02:31 PM, Theodore Ts'o wrote:

On Thu, Feb 26, 2015 at 09:49:39AM +0100, Michael Kerrisk (man-pages) wrote:

quoted

quoted

How about somethign like "This mount significantly reduces writes
needed to update the inode's timestamps, especially mtime and actime.

What is "actime" in the preceding line? Should it be "ctime"?

Sorry, no, it should be "atime".

Thanks.

quoted

I find the wording of there a little confusing. Is the following 
a correct rewrite:

    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.)

Yes, that's correct.  The only other thing I might add is that in the
case of a crash, the atime (or mtime) fields on disk might be out of
date by at most 24 hours.

So in other words, add a sentence to that last para:

    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.

Right?

Cheers,

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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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?

						- Ted