Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

Mark Lord
2009-08-17T20:28:21Z

Greg Freemyer wrote:
..
> Mark, I don't believe your tool really addresses the mdraid situation,
> do you agree. ie. Since your bypassing most of the block stack,
> mdraid has no way of snooping on / adjusting the discards you are
> sending out.
..

Taking care of mounted RAID / LVM filesystems requires in-kernel TRIM
support, possibly exported via an ioctl().

Taking care of unmounted RAID / LVM filesystems is possible in userland,
but would also benefit from in-kernel support, where layouts are defined
and known better than in userland.

The XFS_TRIM was an idea that Cristoph floated, as a concept for examination.

I think something along those lines would be best, but perhaps with an
interface at the VFS layer. Something that permits a userland tool
to work like this (below) might be nearly ideal:

main() {
int fd = open(filesystem_device);
while (1) {
int g, ngroups = ioctl(fd, GET_NUMBER_OF_BLOCK_GROUPS);
for (g = 0; g < ngroups; ++g) {
ioctl(fd, TRIM_ALL_FREE_EXTENTS_OF_GROUP, g);
}
sleep(3600);
}
}

Not all filesystems have a "block group", or "allocation group" structure,
but I suspect that it's an easy mapping in most cases.

With this scheme, the kernel is absolved of the need to track/coallesce
TRIM requests entirely.

Something like that, perhaps.