When we create LVMs or the RAID partitions, the data we save in the parition is divided into PEs or the chunks respectively instead of the conventional blocks of ext3 or ext4.

My doubt is whose responsibility (RAM, Processor, OS) it is to divide the data into PEs or chunks such that the data would be stored on the harddisk in that format.

Moreover is it possible to change the default PE or chunk size? If so, how and when generally we get the requirement of changing the size?

My understanding on LVM PE is as follows: The PE size in LVM is 4MB. Generally a sector is 512 bytes on the hard disk. And on formatting a partition with LVM, each file saved in that partition by default takes 4MB minimum space and continuous sectors and though if any free space is left over in that 4MB it will not allow another file to fit in the same space. The new file again has to go for the new PE. Similar the case with RAID chunks.

Please correct me if I have understood wrongly.


Physical extents in LVM have no relationship to the sizes of files stored within a logical volume. A file in a logical volume does not take a minimum of 4MB (or whatever the LV's chunk size is). As far as the filesystem is concerned, a logical volume is no different from any other block device (such as an ordinary disk partition).

LVM chunks define how the logical volume's block device is built from portions of the underlying block device (aka "physical volume", typically a disk partition). If you were to read the entire contents of a logical volume — the raw block device — you'd get all the bytes in chunk 0, followed by all the bytes in chunk 1, followed by all the bytes in chunk 2, and so on. Those chunks may not be contiguous on the underlying disk partition, but they appear contiguous when you read from the logical volume. (That's pretty much the whole point of LVM.)

So, a 4MB chunk size basically just means that when you allocate space to a logical volume, it has to be done in multiples of 4MB. But that only affects the size of the logical volume itself; when you create a filesystem within it, the filesystem does its own bookkeeping to divide the space up into files, and it doesn't know or care about how the logical volume is actually a sequence of chunks of the underlying physical volume.


LVM and RAID only serve to present a block device to the operating system just like any other disk or partition. In other words they are on the same level as a partition as far as filesystems are concerned.

Whether LVM divides the available space up in 4MB blocks or whatever is not relevant. The filesystem you create on top of that LVM device does not care and will as usual use 4kB blocks for all operations.

Your note "(RAM, Processor, OS)" would indicate you don't really have a grasp on how computer systems work...

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