Re: [PATCH v2] packfile: freshen the mtime of packfile by configuration

[Sun Chao <16657101987@163.com>]

> 2021年7月16日 00:42，Taylor Blau <me@ttaylorr.com> 写道：
> 
> On Fri, Jul 16, 2021 at 12:30:18AM +0800, Sun Chao wrote:
>> I'm sorry to reply so late, I work long hours during the day, and the
>> company network can not send external mail, so I can only go home late
>> at night to reply to you.
> 
> There's no need to apologize :-).
> 
>> Thanks for your reply again, My explaination for 'why the mtime is so
>> important' lost some informations and it is not clear enough, I will
>> tell the details here:
> 
> Let me see if I can summarize here. Basically:
> 
>  - You have a number of servers that have NFS mounts which hold large
>    repositories with packs in excess of 10 GB in size.
>  - You have a lot of clients that are fetching, and a smaller number of
>    clients that are pushing, some of which happen to freshen the mtimes
>    of the packs.
> 
> ...and the mtimes being updated cause the disk cache to be invalidated?
> 
> It's the last part that is so surprising to me. Ævar and I discussed
> earlier in the thread that their understanding was that you had a backup
> system which had to resynchronize an unchanged file because its metadata
> had changed.
> 
> But this is different than that. If I understand what you're saying
> correctly, then you're saying that the disk caches themselves are
> invalidated by changing the mtime.
> 
> That is highly surprising to me, since the block cache should only be
> invalidated if the *blocks* change, not metadata in the inode. It would
> be good to confirm that this is actually what's happening.
> 
> Thanks,
> Taylor

Oh, Maybe I didn't understand caching well enough, let me check it again,
and thanks for your and Ævar's answers, they are really helpful.