linux-ext4 - Re: suspiciously good fsck times?

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Message-ID: <4876468C.2040005@redhat.com>
Date:	Thu, 10 Jul 2008 13:27:40 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Theodore Tso <tytso@....edu>
CC:	Eric Sandeen <sandeen@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: suspiciously good fsck times?

Theodore Tso wrote:
> On Thu, Jul 10, 2008 at 11:14:28AM -0500, Eric Sandeen wrote:
>   
>> Val & I talked about this a little, and came to the conclusion that
>> directory fragmentation might be a pretty big part of it.
>>     
>
> Hmm, could be.  Let's see.  Ric said 46.5 million files, I don't know
> how big the filenames were, but let's assume a directory entry size of
> 32, so that means if we assume perfect packing, 128 directory entries
> per 4k block.  Let's use 100 directory entries/blok just to make the
> math easyer, so that's 465,000 blocks.  If we assume a 10ms seek time,
> and that the blocks are totally scattered, that's 4650 seconds, or
> 1.29 hours. So that's roughly within the ballpark that Ric measured.
>
>      	       	      	      	     	 	  - Ted
>   

(changing cc to the real list instead of ext4 owner - sorry!)

The file names are 40 bytes long, (6 initial bytes of time stamp with 24 
random bytes at end of name. For example:

451aeb61ead89~~~DYASX8LYL4NAUWK3WI187VRP

The 4 threads chose the target subdirectory based on the time stamp, 
rotating into a new subdirectory every 3 minutes or so.

ric

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