File System Selection
sandeen at redhat.com
Wed May 6 15:04:19 UTC 2009
> 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
> 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.
> Thanks in advance.
> Regards, Ramesh
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