linux-ext4 - Re: [PATCH] test-appliance: add new test configuration: ext4/lustre

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Date:   Fri, 23 Jun 2017 19:08:19 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Andreas Dilger <adilger@...ger.ca>, G@...nk.org
Cc:     Artem Blagodarenko <artem.blagodarenko@...gate.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] test-appliance: add new test configuration:
 ext4/lustre_mds

On Fri, Jun 23, 2017 at 04:00:05PM -0600, Andreas Dilger wrote:
> > Are you really using large_dir on a file system that is using indirect
> > block mapped files?
> 
> Like I wrote above, we haven't been using large_dir in production yet.
> My plan was only to use it on the MDT, which is where the filesystem
> namespace is located.  Indeed, we don't use extents on the MDT because:
> a) ext4 MDTs are strictly less than 16TB in size (usually < 8TB, even with
>    4B inodes) because of using "-I 2048" to limit the space per inode,
>    so they never need more than 2^32 blocks
> b) the only files of any size on the MDT are directories or other log
>    files, which are rarely created with contiguous block allocations,
>    so extents usually take more space than indirect blocks (12 bytes vs 4).

OK, I'll try adding large_dir to the luster_mds configuration.  I'm
not sure we're actually going to end up creating a directory with 3
levels of htree nodes, though.  In order to force that we'll probably
need to create a new ext4-specific htree stress tester which creates a
large number of filenames that very long, preferably while using a 1k
block file system.

					- Ted