linux-ext4 - Re: How many files to create in one directory?

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Message-ID: <52E69F3F.2000104@redhat.com>
Date:	Mon, 27 Jan 2014 12:02:39 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Masato Minda <minmin@...s.co.jp>, linux-ext4@...r.kernel.org
Subject: Re: How many files to create in one directory?

On 1/27/14, 1:16 AM, Masato Minda wrote:
> Dear Ext4 Developer;
> 
> I've copied the files from VxFS to EXT4, I saw the message “Directory
> index full!”. I've checked the search engine and found these mails.
> 
>   http://www.spinics.net/lists/linux-ext4/msg25058.html
>   http://www.spinics.net/lists/linux-ext4/msg25069.html
> 
> Our Ext4's block size was 1024. I've changed the block size from 1024 to
> 4096. Our problem was solved. But I have question.
> 
> How many files to create in one directory in 4096 block size?
> I think that it is about 3 million files with perfect hashing.
> 
>   16*(4096-16)/8*(4096-8)/8*3/4= 3M
> 
> Is this correct?

It will depend on the length of the filenames.  But by my calculations,
for average 28-char filenames, it's closer to 30 million.

There are (4096-32)/8 indices per block, or 508.
There are 2 levels, so 508*508=258064 leaf blocks.
The length of each record for 28 char names would be 32 bytes.
So you can fit 4096/32 = 128 entries per leaf block.
258064 leaf blocks * 128 entries/bock is 33,032,192 entries.

I recently made a spreadsheet to calculate this.
I'm not sure if I am doing google docs sharing and protection
correctly, but this might work:

https://docs.google.com/spreadsheet/ccc?key=0AtdHTZsZ8XoYdE1IUXlDb1RXQkdPM3F4YWpfNGhMbFE&usp=sharing#gid=0

-Eric

> Thanks in advance.
> 
> --
> Masato minmin Minda <minmin@...s.co.jp>
> Japan Registry Services Co., Ltd. (JPRS)
> 
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