Hi there,

I would like to do some testing of the ext4 filesystem on a device with a 64KiB block size. Not having one of those to hand, unsurprisingly, I thought I could look into creating a VirtualBox VM hard disk with a 64KiB block size, then installing Linux on it.

Does anybody know how (i) I can specify the block size of a VirtualBox VDI hard disk when I am creating it (or am I constrained by the physical structure of the underlying disk where the VDI is stored?); (ii) Whether I'll have any problems installing Linux on such a device?

Of course there may be alternative ways of emulating a 64KiB block sized device so that mke2fs allows creation of a 64KiB-blocksize ext4 filesystem on it without producing warnings - I'm all ears!

You have to emulate it (use the "-b block-size" of mke2fs).

As far as I know, there are no disks that implement a 64k block size. A RAID controller CAN do that (it would take 16 disks with a 4k native block size) - but even there it still defaults (to my knowledge) to a 4K default (and you want to make the block size the span size of the RAID configuration).

Part of the reason for using that size would be to force preallocation (the smallest file would be one block - 64kb in size)...

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Note that the kernel's ext4 driver does not support block sizes larger than 4096 bytes. That's what the warning is about. You would have to install the fuseext2 package to use such a filesystem.

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That 4k limit is not rigid - it does I/O in page size units which are 4K.

But you can also specify the stripe (https://www.kernel.org/doc/Documenta...stems/ext4.txt)

And there is no specific limitation on the block size.

From what I can find, it appears the only limitations is the system page size...

Yes, and the system page size on x86 is 4096 bytes. If you build a filesystem with a larger block size ("-b" option), the kernel's driver won't be able to mount it. The "stripe=n" parameter is a mount option. You can use that to get allocation and alignment in units larger than the filesystem block size.

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Thanks to both of you.

I've just confirmed that I can force a 64KiB block-sized ext4 filesystem but that the kernel ext4 driver refuses to mount it.

Out of curiosity, did you try to mount using the stripe= option instead?

No, because I wanted to try out the 64KiB block size, as in create, mount and run the filesystem with the actual structure oriented round 64KiB blocks and the superblock field s_log_block_size set to 6. I assumed that the stripe=n mount option would keep the structure at 4KiB and s_log_block_size set at 2, only affecting mballoc's allocation of contiguous blocks, or something like that.

However, to be honest, I don't really understand RAID or strides/stripes.

Did you try the fuseext2 package, which is available for Mint 17? My understanding is that the fuseext2 command should be able to mount that filesystem.