linux-kernel - Re: Zero-clearing all zero-clearable bytes.

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Message-ID: <4928C88B.3040101@lougher.demon.co.uk>
Date:	Sun, 23 Nov 2008 03:05:47 +0000
From:	Phillip Lougher <phillip@...gher.demon.co.uk>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
CC:	adilger@....com, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Zero-clearing all zero-clearable bytes.

Tetsuo Handa wrote:
> Hello.
> 
> Andreas Dilger wrote:
>> Why would you fill the buffer with 0xff instead of 0?
>> In fact no such program is needed, just "dd if=/dev/zero of=/{fs}/tmp"
>> and then delete the file.
>>
> To avoid that the /{fs}/tmp is created as a sparse file.
> 

Most filesystems will not create a sparse file if zero-byte filled 
blocks are written.  To create a sparse file you normally have to seek 
beyond the file end and then write blocks, leaving a hole in-between the 
positions.

The information from stat can tell you if a file has been stored 
sparsely, because the blocks used count will be less than the file size 
suggests.

For example:

$ dd if=/dev/zero of=small-file bs=512 count=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.000187744 seconds, 43.6 MB/s
phillip@...an:/tmp$ stat small-file
   File: `small-file'
   Size: 8192            Blocks: 16         IO Block: 4096   regular file
Device: 801h/2049d      Inode: 847831      Links: 1

The zero filled file uses 16 blocks (16 * 512 bytes = 8K), and so we 
know it isn't sparsely stored.

You can get dd to seek a number of blocks into the output file before 
writing, this will create a sparse file...

$ dd if=/dev/zero of=small-file bs=512 count=16 seek=16
16+0 records in
16+0 records out
8192 bytes (8.2 kB) copied, 0.000212609 seconds, 38.5 MB/s
phillip@...an:/tmp$ stat small-file
   File: `small-file'
   Size: 16384           Blocks: 16         IO Block: 4096   regular file
Device: 801h/2049d      Inode: 847831      Links: 1

A 16K file is only using 16 blocks, and so 8K is stored sparsely.

Phillip
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