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Re: File System Selection



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

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.

>>> 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?

>>> 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.

Thanks in advance.


Regards,
Ramesh



-----Original Message-----
From: "Eric Sandeen" <sandeen redhat com>
Sent: Tuesday, 5 May, 2009 8:53pm
To: "Ramesh" <ramesh arasan com>
Cc: ext3-users redhat com, linux-ext4 vger kernel org
Subject: Re: File System Selection

Ramesh wrote:
> Hi All,
> 
> 
> I am developing a SD Block Driver.
> 
> As per old specification (SD Spec 2.0 ) Maximum size of SD memory
> card is 32 GB. - We used ext2 file system.
> 
> By referring the new Specification (SD Spec 3.0) SD memory card size
> is reached upto and including 2TB (Terra Byte) - Block size strictly
> limited to 512 only (as per specification).
> 
> My Questions.
> 
> 1. For 2TB disk with Block size 512, Which file system is preferred
> (ext3/ext4)

do you mean sector size of the block device, or block size of the
fileystem?  I guess it doesn't matter much either way, 2^32*512 is 2T.
Either ext3 or ext4 can handle this size, you'll probably need to make
your decision based on other factors.

> 2. In a 32 bit machine, If I installed the Fedora 10 ( having ext4),
> am I able to use it as effectively ( for the maximum disk/file size
> usage). To utilize 2TB or more size hard disk, is this allowable to
> use 32 bit machine with Ext4 fs?
> 

On a 32 bit machine you will be limited to 16T, this is actually a page
cache limitation.  But 2T should be fine.

-Eric


> Thanks in advance.
> 
> Regards, Ramesh
> 


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