Re: [PATCH 5/9] spaceman/defrag: exclude shared segments on low free space

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jul 11, 2024 at 11:08:39PM +0000, Wengang Wang wrote:
> 
> 
> > On Jul 9, 2024, at 2:05 PM, Darrick J. Wong <djwong@kernel.org> wrote:
> > 
> > On Tue, Jul 09, 2024 at 12:10:24PM -0700, Wengang Wang wrote:
> >> On some XFS, free blocks are over-committed to reflink copies.
> >> And those free blocks are not enough if CoW happens to all the shared blocks.
> > 
> > Hmmm.  I think what you're trying to do here is avoid running a
> > filesystem out of space because it defragmented files A, B, ... Z, each
> > of which previously shared the same chunk of storage but now they don't
> > because this defragger unshared them to reduce the extent count in those
> > files.  Right?
> > 
> 
> Yes.
> 
> > In that case, I wonder if it's a good idea to touch shared extents at
> > all?  Someone set those files to share space, that's probably a better
> > performance optimization than reducing extent count.
> 
> The question is that:
> Are the shared parts are something to be overwritten frequently?
> If they are, Copy-on-Write would make those shared parts fragmented.
> In above case we should dedefrag those parts, otherwise, the defrag might doesn’t defrag at all.
> Otherwise the shared parts are not subjects to be overwritten frequently,
> They are expected to remain in big extents, choosing proper segment size
> Would skip those.
> 
> But yes, we can add a option to simply skip those share extents. 

Good enough for now, I think. :)

> > 
> > That said, you /could/ also use GETFSMAP to find all the other owners of
> > a shared extent.  Then you can reflink the same extent to a scratch
> > file, copy the contents to a new region in the scratch file, and use
> > FIEDEDUPERANGE on each of A..Z to remap the new region into those files.
> > Assuming the new region has fewer mappings than the old one it was
> > copied from, you'll defragment A..Z while preserving the sharing factor.
> 
> That’s not safe? Things may change after GETFSMAP.

It is if after you reflink the same extent to a scratch file, you then
check that what was reflinked into that scratch file is the same space
that you thought you were cloning.  If not, truncate the scratch file
and try the GETFSMAP again.

The dedupe should be safe because it doesn't remap unless the contents
match.

--D

> > 
> > I say that because I've written such a thing before; look for
> > csp_evac_dedupe_fsmap in
> > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/commit/?h=defrag-freespace&id=785d2f024e31a0d0f52b04073a600f9139ef0b21
> > 
> >> This defrag tool would exclude shared segments when free space is under shrethold.
> > 
> > "threshold"
> 
> OK.
> 
> Thanks
> Wengang
> > 
> > --D
> > 
> >> Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
> >> ---
> >> spaceman/defrag.c | 46 +++++++++++++++++++++++++++++++++++++++++++---
> >> 1 file changed, 43 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/spaceman/defrag.c b/spaceman/defrag.c
> >> index 61e47a43..f8e6713c 100644
> >> --- a/spaceman/defrag.c
> >> +++ b/spaceman/defrag.c
> >> @@ -304,6 +304,29 @@ void defrag_sigint_handler(int dummy)
> >> printf("Please wait until current segment is defragmented\n");
> >> };
> >> 
> >> +/*
> >> + * limitation of filesystem free space in bytes.
> >> + * when filesystem has less free space than this number, segments which contain
> >> + * shared extents are skipped. 1GiB by default
> >> + */
> >> +static long g_limit_free_bytes = 1024 * 1024 * 1024;
> >> +
> >> +/*
> >> + * check if the free space in the FS is less than the _limit_
> >> + * return true if so, false otherwise
> >> + */
> >> +static bool
> >> +defrag_fs_limit_hit(int fd)
> >> +{
> >> + struct statfs statfs_s;
> >> +
> >> + if (g_limit_free_bytes <= 0)
> >> + return false;
> >> +
> >> + fstatfs(fd, &statfs_s);
> >> + return statfs_s.f_bsize * statfs_s.f_bavail < g_limit_free_bytes;
> >> +}
> >> +
> >> /*
> >>  * defragment a file
> >>  * return 0 if successfully done, 1 otherwise
> >> @@ -377,6 +400,15 @@ defrag_xfs_defrag(char *file_path) {
> >> if (segment.ds_nr < 2)
> >> continue;
> >> 
> >> + /*
> >> + * When the segment is (partially) shared, defrag would
> >> + * consume free blocks. We check the limit of FS free blocks
> >> + * and skip defragmenting this segment in case the limit is
> >> + * reached.
> >> + */
> >> + if (segment.ds_shared && defrag_fs_limit_hit(defrag_fd))
> >> + continue;
> >> +
> >> /* to bytes */
> >> seg_off = segment.ds_offset * 512;
> >> seg_size = segment.ds_length * 512;
> >> @@ -478,7 +510,11 @@ static void defrag_help(void)
> >> "can be served durning the defragmentations.\n"
> >> "\n"
> >> " -s segment_size    -- specify the segment size in MiB, minmum value is 4 \n"
> >> -"                       default is 16\n"));
> >> +"                       default is 16\n"
> >> +" -f free_space      -- specify shrethod of the XFS free space in MiB, when\n"
> >> +"                       XFS free space is lower than that, shared segments \n"
> >> +"                       are excluded from defragmentation, 1024 by default\n"
> >> + ));
> >> }
> >> 
> >> static cmdinfo_t defrag_cmd;
> >> @@ -489,7 +525,7 @@ defrag_f(int argc, char **argv)
> >> int i;
> >> int c;
> >> 
> >> - while ((c = getopt(argc, argv, "s:")) != EOF) {
> >> + while ((c = getopt(argc, argv, "s:f:")) != EOF) {
> >> switch(c) {
> >> case 's':
> >> g_segment_size_lmt = atoi(optarg) * 1024 * 1024 / 512;
> >> @@ -499,6 +535,10 @@ defrag_f(int argc, char **argv)
> >> g_segment_size_lmt);
> >> }
> >> break;
> >> + case 'f':
> >> + g_limit_free_bytes = atol(optarg) * 1024 * 1024;
> >> + break;
> >> +
> >> default:
> >> command_usage(&defrag_cmd);
> >> return 1;
> >> @@ -516,7 +556,7 @@ void defrag_init(void)
> >> defrag_cmd.cfunc = defrag_f;
> >> defrag_cmd.argmin = 0;
> >> defrag_cmd.argmax = 4;
> >> - defrag_cmd.args = "[-s segment_size]";
> >> + defrag_cmd.args = "[-s segment_size] [-f free_space]";
> >> defrag_cmd.flags = CMD_FLAG_ONESHOT;
> >> defrag_cmd.oneline = _("Defragment XFS files");
> >> defrag_cmd.help = defrag_help;
> >> -- 
> >> 2.39.3 (Apple Git-146)
> >> 
> >> 
> > 
>