Re: btrfs: 21 minutes to read 1.2M file directory

[Hugo Mills]

On Wed, Dec 22, 2010 at 12:39:15PM -0800, Andy Isaacson wrote:
> On Tue, Dec 21, 2010 at 03:07:33AM +0200, Felipe Contreras wrote:
> > On Tue, Dec 21, 2010 at 12:24 AM, Andy Isaacson <adi@xxxxxxxxxxxxx> wrote:
> > > I have a directory with 1.2M files in it, which makes readdir very slow
> > > on btrfs with cold caches (although it's reasonably fast with hot caches
> > > as in the first example below):
> > 
> > Sounds like:
> > 
> > Bug 21562 - btrfs is dead slow due to fragmentation
> > https://bugzilla.kernel.org/show_bug.cgi?id=21562
> 
> Hmmm, how do I look at the btree layout for a given inode?

   There's documentation on the tree structures at [1] and [2]. If you
know the inode number of the object you're interested in, you need to
look in the FS tree for the subvolume it's in and find the
(inode_number, EXTENT_DATA, ...) keys for the file. Each of those
records will reference an individual disk extent -- and you can get
the disk start position and length of the extent from the data stored
under the key.

   Hugo.

[1] https://btrfs.wiki.kernel.org/index.php/Btree_Items
[2] https://btrfs.wiki.kernel.org/index.php/Data_Structures

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