Re: zerofree btrfs support?

[Adam Borowski <kilobyte@angband.pl>]

On Sat, Mar 10, 2018 at 07:37:22PM +0500, Roman Mamedov wrote:
> Note you can use it on HDDs too, even without QEMU and the like: via using LVM
> "thin" volumes. I use that on a number of machines, the benefit is that since
> TRIMed areas are "stored nowhere", those partitions allow for incredibly fast
> block-level backups, as it doesn't have to physically read in all the free
> space, let alone any stale data in there. LVM snapshots are also way more
> efficient with thin volumes, which helps during backup.

Since we're on a btrfs mailing list, if you use qemu, you really want
sparse format:raw instead of qcow2 or preallocated raw.  This also works
great with TRIM.

> > Back then it didn't seem to work.
> 
> It works, just not with some of the QEMU virtualized disk device drivers.
> You don't need to use qemu-img to manually dig holes either, it's all
> automatic.

It works only with scsi and virtio-scsi drivers.  Most qemu setups use
either ide (ouch!) or virtio-blk.

You'd obviously want virtio-scsi; note that defconfig enables virtio-blk but
not virtio-scsi; I assume most distribution kernels have both.  It's a bit
tedious to switch between the two as -blk is visible as /dev/vda while -scsi
as /dev/sda.


Meow!
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