On Fri, 20 Sep 2024, Steve Keller wrote:
I'd like to understand some technical details about how fstrim, file
systems, and block devices work.
Do ext4 and btrfs keep a list of blocks that have already been reported as
unused or do they have to report all unused blocks to the block device
layer everytime the fstrim command is issued?
Does LVM keep information on every block about its usage or does it always
have to pass trim operations to the lower layer?
And does software RAID, i.e. /dev/md* keep this information on every block?
Can RAID skip unused blocks from syncing in a RAID-1 array when I replace a
disk?
Steve
In the default, iscsi, md, lvm, ext2 do not keep this information. Don't
know if it's configurable sonewhere but I suspect not. Don't know about
btrfs.
Some of this data is cached, but not between reboots.
The raid rebuild is a particular pain point IMO. It's important to do a
discard after a failed disk rebuild otherwise every block is 'in use' on
the underlying storage.
After a rebuild I always create a LV with all the free space and then
discard it.
I think a VG free space skipping md rebuild would suit me better than
discard tracking at all the different levels. I guess ZFS users might
have a different view of how useful lvm aware mdraid is :-)
