On 2018-04-20 10:21, David Sterba wrote:
This patchset adds new ioctl similar to TRIM, that provides several
other ways how to clear the unused space. The changelogs are
incomplete, for preview not for inclusion yet.
+1 for the idea. This will be insanely useful for certain VM setups.
It compiles and has been tested lightly, the clearing modes depend on hw
capabilities (secure discard, sector unmapping instead of zeros), so
I've tested only zeroing and discard.
I don't have specific experience with blkdev_issue_zeroout, but I think
the sector unmapping may be covered by the discard support. When
dealing with SCSI devices, discard operations get converted to the SCSI
UNMAP command, which just based on the naming, sounds like what
blkdev_issue_zeroout is issuing (at least, when dealing with SCSI
devices). Pretty much, the SCSI command works just like the ATA TRIM
command does on ATA devices which zero data on discard (except it's
SCSI, so it can pretty much always be queued, and has some other
semantics around it that give it better performance than ATA TRIM).
Assuming that is the case, the unmapping support could be tested using
the QEMU PV-SCSI driver (which supports the UNMAP command) with
appropriate backing storage. I can look at doing this testing myself,
as I already use PV-SCSI for storage on all my QEMU VM's.
I personally think the zeroing has a usecase and the other modes were
easy to add. Further extensions can be considered, eg. WRITE_SAME,
overwriting with a randomly generated pattern, or some filesystem canary
patterns that can be used to report unused block read as metadata.
Zeroing does have a use-case. It's wonderfully useful for handling
compaction of VM disks when using a virtual machine manager that doesn't
support discard operations for this purpose. I can also see the secure
discard support being useful (though not necessarily the sector
unmapping support for reasons mentioned above).
As this is modelled after the generic FITRIM ioctl, so this could be a
new generic ioctl too. However, last time somebody wanted such ioctl,
there was a pushback. I'll consider making a generic version and send it
for comments to fsdevel eventually.
git://github.com/kdave/btrfs-devel dev/zero-free
David Sterba (6):
btrfs: extend trim callchain to pass the operation type
btrfs: add wrapper to switch clearing operation
btrfs: add zeroing clear operation
btrfs: add new ioctl BTRFS_IOC_CLEAR_FREE
btrfs: add discard secure to clear unused space
btrfs: add more zeroout modes for clearing unused space
fs/btrfs/ctree.h | 5 +-
fs/btrfs/extent-tree.c | 130 ++++++++++++++++++++++++++++++++++++++------
fs/btrfs/free-space-cache.c | 22 +++++---
fs/btrfs/free-space-cache.h | 3 +-
fs/btrfs/ioctl.c | 42 ++++++++++++++
include/uapi/linux/btrfs.h | 26 +++++++++
6 files changed, 199 insertions(+), 29 deletions(-)
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