Re: [PATCH v5 25/27] refs: add LMDB refs storage backend

> On Sat, 2016-02-20 at 15:59 +0700, Duy Nguyen wrote:
>> On Thu, Feb 18, 2016 at 12:17 PM, David Turner <
>> dturner@twopensource.com> wrote:
>> > LMDB has a few features that make it suitable for usage in git:
>> > ...
>>
>> I'm reading lmdb documents and hitting  the caveat section [1].
>> Random thoughts
>>
>> * "There is normally no pure read-only mode, since readers need write
>> access to locks and lock file.".
>>
>> This will be a problem for server side that serves git:// protocol
>> only. Some of those servers disable write access to the entire
>> repository and git still works fine (but won't when lmdb is used).
>> Should we do something in this case? Just tell server admins to relax
>> file access, or use MDB_NOLOCK (and when? based on config var?)
>
> MDB_NOLOCK is a good idea. I'm going to add this to the "Weaknesses"
> section of the docs and plan to fix it later, unless you feel strongly
> otherwise.

>> * "Avoid long-lived transactions...."
>>
>> OK we don't have a problem with this. But it makes me realize lmdb
>> transactions do not map with ref transactions. We don't open lmdb
>> transaction at ref_transaction_begin(), for example. Is it simply
>> more
>> convenient to do transactions the current way, or is it impossible or
>> incorrect to attach lmdb transactions to ref_transaction_*()?
>
> That was what I did originally, but reviewers here noted that it had
> some problems:
> 1. What if, while a transaction is open, git opens a subprocess that
> wants to make its own transaction?  There can only be one writer
> transaction open at a time.
> 2. As you note, long-lived transactions.
>
> Also, the files backend also doesn't do any locking until the last
> moment, and it's reasonable to try to be compatible with that.