Re: [RFC] Live resize of backing device

[Coly Li <colyli@suse.de>]

> 2023年2月3日 04:48，Eric Wheeler <bcache@lists.ewheeler.net> 写道：
> 
> On Fri, 3 Feb 2023, Coly Li wrote:
>>> 2023年1月27日 20:44，Andrea Tomassetti <andrea.tomassetti-opensource@devo.com> 写道：
>>> From 83f490ec8e81c840bdaf69e66021d661751975f2 Mon Sep 17 00:00:00 2001
>>> From: Andrea Tomassetti <andrea.tomassetti-opensource@devo.com>
>>> Date: Thu, 8 Sep 2022 09:47:55 +0200
>>> Subject: [PATCH v2] bcache: Add support for live resize of backing devices
>>> 
>>> Signed-off-by: Andrea Tomassetti <andrea.tomassetti-opensource@devo.com>
>>> ---
>>> Hi Coly,
>>> this is the second version of the patch. As you correctly pointed out,
>>> I implemented roll-back functionalities in case of error.
>>> I'm testing this funcionality using QEMU/KVM vm via libvirt.
>>> Here the steps:
>>> 1. make-bcache --writeback -B /dev/vdb -C /dev/vdc
>>> 2. mkfs.xfs /dev/bcache0
>>> 3. mount /dev/bcache0 /mnt
>>> 3. dd if=/dev/random of=/mnt/random0 bs=1M count=1000
>>> 4. md5sum /mnt/random0 | tee /mnt/random0.md5
>>> 5. [HOST] virsh blockresize <vm-name> --path <disk-path> --size <new-size>
>>> 6. xfs_growfs /dev/bcache0
>>> 6. Repeat steps 3 and 4 with a different file name (e.g. random1.md5)
>>> 7. umount/reboot/remount and check that the md5 hashes are correct with
>>>       md5sum -c /mnt/random?.md5
>> 
>> [snipped]
>> 
>> Hi Andrea,
>> 
>> For the above step 5, could you provide a specific command line for me to reproduce?
>> 
>> I tried to resize the disk by virsh from 400G to 800G, the disk resize 
>> doesn’t happen online, I have to stop and restart the virtual machine 
>> and see the resized disk.
> 
> Coly, 
> 
> The `virsh blockresize` command only informs the vm that the size has 
> changed. You will need to resize the disk itself using `lvresize` or 
> whatever your VM's backing device is. If you're backing device for the 
> virtual machine is a file, then you could use `truncate -s <size> 
> /file/path` before issuing `virsh blockresize`.
> 
> Another possibility: are you using virtio-blk as shown in his example, or 
> a different virtual driver like virtio-scsi?
> 

Yes, it was virtio-scsi drive.


> If the VM doesn't detect it you might need to do this if its a 
> virtio-scsi disk on the inside:
> 
> for i in /sys/class/scsi_host/*/scan; do echo - - - > $i; done
> 
> If you're using virtio-blk (vda) then it should be immediate.  I don't 
> think IDE will work at all, and not sure about nvme.

Sure, now I switch to virtio-blk drive and I can see the backing device size extends immediately after blockresize command in virsh.

Thanks.

Coly Li