[otterley]

I’m a bit confused about this:

“The storage device driver exposes Fluid Storage volumes as standard Linux block devices mountable with filesystems such as ext4 or xfs. It...allows volumes to be resized dynamically while online.”

Yet an xfs file system cannot be shrunk at all, and an ext4 filesystem cannot be shrunk without first unmounting it.

Are you simply doing thin provisioning of these volumes, so they appear to be massive but aren’t really? I see later that you say you account for storage based on actual consumption.

[namibj]

..... They are block devices. Those allow resizing.

They can be used with, for example, the listed file systems.

No one claimed the listed file systems would (usefully) cooperate with (all aspects of) the block device's resizing.

[otterley]

You can’t just shrink a block device any way you want. It has to be done in concert with the filesystem on top of it to ensure any allocated blocks in the space to be deallocated are relocated to a safe area in the volume. Otherwise, data could be lost and filesystem corruption could occur.

Put differently, there is no point in being able to shrink a volume if you can’t safely shrink the filesystem that uses it.

The usual solution to this problem is thin provisioning, where you put a translation layer between the blocks the filesystem thinks it’s using and the actual underlying blocks. With thin provisioning you can allocate only, say, 1GB to the physical storage, but the block device presents itself as much larger than that, so you can pretend to create a 1PB filesystem on top of it.