linux-ext4 - Re: File System Selection

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Message-ID: <4A01A6F3.9010306@redhat.com>
Date:	Wed, 06 May 2009 10:04:19 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Ramesh <ramesh@...san.com>
CC:	ext3-users@...hat.com, linux-ext4@...r.kernel.org,
	A Garg <agarg@...san.com>, Prashant <prashant@...san.com>,
	Sridevi <sridevi@...san.com>
Subject: Re: File System Selection

Ramesh wrote:
> Hi Eric,
> 
> Thanks for your prompt and informative reply.
> 
>>>> do you mean sector size of the block device, or block size of
>>>> the fileystem?
> For our device sector size is 4906 bytes. But the maximum allowed
> data chunk to read/write is 512( a.k.a Block size), restricted by
> specification.
> 
> By referring the wiki pages of EXT3
> (http://en.wikipedia.org/wiki/Ext3), I saw the below table.
> 
> Block size       Max file size  Max filesystem size
> 1 KiB            16 GiB         <2 TiB
> 2 KiB            256 GiB        <4 TiB
> 4 KiB            2 TiB          <8 TiB
> 8 KiB[limits 1]  2 TiB          <16 TiB

Above, block size means the filesystem block size.

For ext3, all 32 bits should be safe on recent kernels and userspace, so
I think the max filesystem sizes listed above are too small by half.

IOW, 4k filesystem blocks -> 16T max filesystem size.

> And by taking the values with the table, then for 512 bytes block
> size, Max file system supported is 1 TB only. Please correct me, if I
> assumed wrongly.

you cannot have a 512 byte block size in ext3, 1k is the minimum.

>>>> I guess it doesn't matter much either way, 2^32*512 is 2T.
> 
> In that 32 bit, it using the MSB as signed bit. So it can use maximum
> of 31 bits only. Is this correct?

all 32 bits should be safe now.

>>>> On a 32 bit machine you will be limited to 16T, this is
>>>> actually a page cache limitation.  But 2T should be fine.
> 
> Please clarify me that Ext4 is using a 48 bit addressing. Is this
> necessary to go for 64 bit machines to utilize Ext4 and manage up to
> and including 2TB size file system... Please clarify me.

The ext4 ondisk format does use 48 bits for physical addressing, but
userspace is still 32 bits only even for ext4.

-Eric

> Thanks in advance.
> 
> 
> Regards, Ramesh
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