[The context of this is trying to get virt-sparsify to work in place on disks.] On Tue, Mar 11, 2014 at 04:47:02PM -0500, Eric Sandeen wrote: > On 3/11/14, 4:39 PM, Richard W.M. Jones wrote: > > > > Here's a problem I can't work out: > > > > I have a filesystem (in a VM) that I know has at least 100MB of > > deleted files on it. > > Was it mounted with -o discard at the time the files were deleted? No, it was not. I know that the original 'rm' command didn't recover any space because the disk image grew by ~100 MB. > If so, then the trim is already done during the unlink process, > and there's no more work to do. > > So that's my first thought, but ... > > > Doing this in a script: > > > > mount -o discard /dev/sda1 /mnt > > fstrim /mnt > > > > ... does nothing. Also the fstrim is almost instantaneous -- there's > > no way it could be scanning the disk. > > blktrace would be a better tool to find out whether or not discards > are actually getting issued to storage... > > And if you strace it what does the ioctl return? I'll try that in a few minutes. In the mean time I captured the fstrim -v output: fstrim -v / /: 124 MiB (130039808 bytes) trimmed 124 MB is (within 25%) the amount of data I would expect needs to be trimmed. > Enabling the trace_ext4_trim_all_free tracepoint might be interesting too. That a systemtap thing? It's tricky to get systemtap working in a virtual machine, but I guess I can try if nothing else works. > > However, if I start with the same filesystem, mounted with -o discard, > > and create and rm large files, while observing the size of the > > underlying virtual disk, then discard is obviously working fine. 'rm' > > of large files makes the underlying disk shrink. > > > > Any ideas here? > > first of all, I should point out that "-o discard" is not necessary for > fstrim / FITRIM ioctl to work. The former tries to trim as soon > as files are unlinked; FITRIM goes looking for free blocks to trim. > > If you're mounting with -o discard, then fstrim should never find any > workd to do. Useful to know. I thought I had to use -o discard in order for the ext4 module to send discard commands at all to the block layer. Thanks, Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/ -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
