linux-ext4 - Re: [PATCH] Ext4: fix slow writeback under dioread

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Message-Id: <F65698E8-8FFD-4C79-98EF-9971FE5719EB@gmail.com>
Date:   Wed, 21 Nov 2018 19:40:23 +0300
From:   Artem Blagodarenko <artem.blagodarenko@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Liu Bo <bo.liu@...ux.alibaba.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] Ext4: fix slow writeback under dioread_nolock and
 nodelalloc

Hello Theodore,

> On 21 Nov 2018, at 03:07, Theodore Y. Ts'o <tytso@....edu> wrote:
> 
> *) Drop support for nodelalloc, and make delayed allocation the
>   default.  (This requires dropping data=journal mode, since
>   data=journal mode does not co-exist with delalloc.)
> 
> The last was assuming that there really wasn't good use cases where
> people would really want to use nodelalloc, aside from ext3
> bug-for-bug compatibility, and hopefully since most people are using
> ext4 these days, it becomes easier to make the case tha we don't need
> to support ext3's unusual performance attributes.

Currently Lustre FS uses “nodelalloc” option at least because of this two reasons:

- OSD (layer that interact with EXT4 (LDISKFS backend) expect synchronous behaviour.

- delalloc optimise block allocation the way useless for Lustre FS, because data is striped across of OSTs. 
Lustre FS has its own optimisations. Its behaviour similar with data=journal journaling mode. “Journal” is stored on clients and OST recovery is like journal replay.

Best regards,
Artem Blagodarenko.