From: Duy Nguyen <pclouds@gmail.com>
To: David Turner <dturner@twopensource.com>
Cc: Git Mailing List <git@vger.kernel.org>,
Michael Haggerty <mhagger@alum.mit.edu>
Subject: Re: [PATCH v5 25/27] refs: add LMDB refs storage backend
Date: Thu, 25 Feb 2016 17:12:32 +0700 [thread overview]
Message-ID: <CACsJy8DDVbz9LnX5HM_2HB+uDvqzS7LU=12gxhxJFqYPHp0Tvg@mail.gmail.com> (raw)
In-Reply-To: <1456346242.18017.16.camel@twopensource.com>
On Thu, Feb 25, 2016 at 3:37 AM, David Turner <dturner@twopensource.com> wrote:
> 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.
No I'm fine as long as it's documented. I was a bit disappointed when
I found out about this because after reading lmdb paper I almost
suggested we add lmdb to git.git as a submodule and prepare it to be
the next default ref backend. The quest for the "next generation"
default ref backend continues.
>> * "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.
Some of these should be kept in the commit message for future reference.
--
Duy
next prev parent reply other threads:[~2016-02-25 10:13 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-18 5:17 [PATCH v5 00/27] refs backends David Turner
2016-02-18 5:17 ` [PATCH v5 01/27] refs: Move head_ref{,_submodule} to the common code David Turner
2016-02-18 5:17 ` [PATCH v5 02/27] refs: move for_each_*ref* functions into " David Turner
2016-02-18 5:17 ` [PATCH v5 03/27] refs: add a backend method structure with transaction functions David Turner
2016-02-18 5:17 ` [PATCH v5 04/27] refs: add methods for misc ref operations David Turner
2016-02-18 5:17 ` [PATCH v5 05/27] refs: add method for do_for_each_ref David Turner
2016-02-18 5:17 ` [PATCH v5 06/27] refs: add do_for_each_per_worktree_ref David Turner
2016-02-18 5:17 ` [PATCH v5 07/27] refs: add methods for reflog David Turner
2016-02-18 5:17 ` [PATCH v5 08/27] refs: add method for initial ref transaction commit David Turner
2016-02-18 5:17 ` [PATCH v5 09/27] refs: add method for delete_refs David Turner
2016-02-18 5:17 ` [PATCH v5 10/27] refs: add methods to init refs db David Turner
2016-02-18 5:17 ` [PATCH v5 11/27] refs: add method to rename refs David Turner
2016-02-18 5:17 ` [PATCH v5 12/27] refs: forbid cross-backend ref renames David Turner
2016-02-20 4:30 ` Duy Nguyen
2016-02-24 20:48 ` David Turner
2016-02-18 5:17 ` [PATCH v5 13/27] refs: make lock generic David Turner
2016-02-18 5:17 ` [PATCH v5 14/27] refs: move duplicate check to common code David Turner
2016-02-18 5:17 ` [PATCH v5 15/27] refs: allow log-only updates David Turner
2016-02-18 5:17 ` [PATCH v5 16/27] refs: don't dereference on rename David Turner
2016-02-18 5:17 ` [PATCH v5 17/27] refs: on symref reflog expire, lock symref not referrent David Turner
2016-02-18 5:17 ` [PATCH v5 18/27] refs: resolve symbolic refs first David Turner
2016-02-18 5:17 ` [PATCH v5 19/27] refs: always handle non-normal refs in files backend David Turner
2016-02-18 5:17 ` [PATCH v5 20/27] init: allow alternate ref strorage to be set for new repos David Turner
2016-02-18 5:17 ` [PATCH v5 21/27] refs: check submodules' ref storage config David Turner
2016-02-18 5:17 ` [PATCH v5 22/27] clone: allow ref storage backend to be set for clone David Turner
2016-02-18 5:17 ` [PATCH v5 23/27] svn: learn ref-storage argument David Turner
2016-02-20 23:55 ` Eric Wong
2016-02-23 18:08 ` David Turner
2016-02-18 5:17 ` [PATCH v5 24/27] refs: add register_ref_storage_backends() David Turner
2016-02-18 5:17 ` [PATCH v5 25/27] refs: add LMDB refs storage backend David Turner
2016-02-18 8:50 ` Duy Nguyen
2016-02-18 20:23 ` David Turner
2016-02-18 21:15 ` Junio C Hamano
2016-02-19 2:54 ` Duy Nguyen
2016-02-19 19:10 ` David Turner
2016-02-20 13:14 ` Duy Nguyen
2016-02-24 20:41 ` David Turner
2016-02-20 21:32 ` Junio C Hamano
2016-02-19 22:49 ` David Turner
2016-02-19 23:08 ` Junio C Hamano
2016-02-20 2:58 ` Duy Nguyen
2016-02-24 20:43 ` David Turner
2016-02-25 10:07 ` Duy Nguyen
2016-02-20 8:59 ` Duy Nguyen
2016-02-24 20:37 ` David Turner
2016-02-25 10:12 ` Duy Nguyen [this message]
2016-02-25 20:05 ` [PATCH] refs: document transaction semantics David Turner
2016-02-25 20:10 ` David Turner
2016-02-25 20:34 ` Junio C Hamano
2016-02-25 20:50 ` David Turner
2016-02-18 5:17 ` [PATCH v5 26/27] refs: tests for lmdb backend David Turner
2016-02-18 5:17 ` [PATCH v5 27/27] tests: add ref-storage argument David Turner
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