Re: [PATCH 1/3] tmpfs: revert SEEK_DATA and SEEK_HOLE

[Jeff Liu <jeff.liu@oracle.com>]

On 07/16/2012 05:28 PM, Hugh Dickins wrote:

> On Thu, 12 Jul 2012, Jeff Liu wrote:
>> On 07/12/2012 07:01 AM, Dave Chinner wrote:
>>> On Wed, Jul 11, 2012 at 11:55:34AM -0700, Hugh Dickins wrote:
>>>>
>>>> But your vote would count for a lot more if you know of some app which
>>>> would really benefit from this functionality in tmpfs: I've heard of none.
>>>
>>> So what? I've heard of no apps that use this functionality on XFS,
>>> either, but I have heard of a lot of people asking for it to be
>>> implemented over the past couple of years so they can use it.
>>> There's been patches written to make coreutils (cp) make use of it
>>> instead of parsing FIEMAP output to find holes, though I don't know
>>> if that's gone beyond more than "here's some patches"...
>>
>> Yes, for apps, cp(1) will make use of it to replace the old FIEMAP for efficient sparse file copy.
>> I have implemented an extent-scan module to coreutils a few years ago,
>> http://fossies.org/dox/coreutils-8.17/extent-scan_8c_source.html
> 
> Thanks for confirming Dave's pointer to cp.
> 
> Of course, tmpfs has never supported FIBMAP or FIEMAP;
> but SEEK_DATA and SEEK_HOLE do fit it much more naturally.
> 
>>
>> It does extent scan through FIEMAP, however, SEEK_DATA/SEEK_HOLE is more convenient and easy to use
>> considering the call interface.  So FIEMAP will be replaced by SEEK_XXX once it got supported by EXT4.
>>
>> Moreover, I have discussed with Jim who is the coreutils maintainer previously, He would like to post
>> extent-scan module to Gnulib so that other GNU utilities which are relied on Gnulib might be a potential
>> user of it, at least, GNU tar will definitely need it for sparse file backup.
> 
> Thanks for the info.  I confess I'm not hugely swayed by cp and sparse
> file archive arguments - I doubt many people care, and I doubt those who
> do care are using tmpfs for them.
> 
> But my doubts are just ignorance.  I was hoping to hear, not that we have
> tools to copy sparse files efficiently (umm, over the network?), but
> what apps are actually working live with those sparse files on tmpfs,
> and now need to seek around them.  Some math or physics applications?
> 
>>>
>>> Besides, given that you can punch holes in tmpfs files, it seems
>>> strange to then say "we don't need a method of skipping holes to
>>> find data quickly"....
>>
>> So its deserve to keep this feature working on tmpfs considering hole punch. :)
> 
> Well, thank you, as I said earlier I am on both sides of the argument.
> (And feel uncomfortably like a prima donna waiting in the wings until
> the audience has shouted long enough for the encore.)

Oh, sorry, I missed you response to Dave before.

> 
> It's now taken out of 3.5, but we can bring it back when there's more
> demand.

Yep. :)

Thanks,
-Jeff

> Your extent-scan is itself waiting for ext4 to support it:
> maybe get noisy at me when that's imminent.

> 
> Hugh
> 
>>
>> Thanks,
>> -Jeff
>>
>>>
>>> Besides, seek-hole/data is still shiny new and lots of developers
>>> aren't even aware of it's presence in recent kernels. Removing new
>>> functionality saying "no-one is using it" is like smashing the egg
>>> before the chicken hatches (or is it cutting of the chickes's head
>>> before it lays the egg?).
>>>
>>> Cheers,
>>>
>>> Dave.
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