#StackBounty: #linux #raid #ssd #software-raid SSD/NVME friendly MD RAID1 rebuild in Linux

Bounty: 50

I use SSD and NVME RAID1 arrays to store mostly virtual machine disks. More than 75% of the data are zeros (preallocated images, free space).

If a disk fails and gets replaced, the rebuild copies and writes all the data to the replacement disk, which causes thermal throttling in NVME, and I assume more wear on the SSD/NVME. Is there a way to configure the rebuild to compare data from both disks first and write to the new disk only if needed?

Or are SSD/NVME chips supposed to check whether the data are just zeros and if the target blocks are not written yet (giving zeros on read), it would just discard the data without wasting write cycles?
Or if there is any target data should it just trim the block to produce zeros?

I found an old thread at https://www.spinics.net/lists/raid/msg57529.html
but it did not provide an answer.

I tried a workaround but I think it is ugly plus the RAID must be offline.

mdadm --fail /dev/md0 /dev/sde
mdadm -r /dev/md0 /dev/sde

(replace /dev/sde)

mdadm -S /dev/md0
ddpt if=/dev/sdd of=/dev/sde verbose=1 oflag=sparing
mdadm -C -v /dev/md0 --assume-clean -l 1 -n 2 /dev/sdd /dev/sde

Any ideas for compare-write RAID1 rebuild? Thanks.


Get this bounty!!!

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