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roots: add scan-mode 2 "oldest crawler first"

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Add a third scan mode with alternative trade-offs.

Benefits:  Good sequential read performance.  Avoids race conditions
described in https://github.com/Zygo/bees/issues/27.  Avoids diverting
scan resources into short-lived snapshots before their long-lived
origin subvols are fully scanned.

Drawbacks:  Takes the longest time of the three implemented scan-modes
to free space in extents that are shared between snapshots.  Uses the
maximum amount of temporary space.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2018-01-28 00:55:43 -05:00
parent ef44947145
commit 636328fdc2
2 changed files with 27 additions and 0 deletions
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26
src/bees-roots.cc
View File

@ -56,6 +56,7 @@ BeesRoots::scan_mode_ntoa(BeesRoots::ScanMode mode)
static const bits_ntoa_table table[] = {
NTOA_TABLE_ENTRY_ENUM(SCAN_MODE_ZERO),
NTOA_TABLE_ENTRY_ENUM(SCAN_MODE_ONE),
NTOA_TABLE_ENTRY_ENUM(SCAN_MODE_TWO),
NTOA_TABLE_ENTRY_ENUM(SCAN_MODE_COUNT),
NTOA_TABLE_ENTRY_END()
};
@ -262,6 +263,7 @@ BeesRoots::crawl_roots()
}
switch (s_scan_mode) {
case SCAN_MODE_ZERO: {
// Scan the same inode/offset tuple in each subvol (good for snapshots)
BeesFileRange first_range;
@ -289,6 +291,7 @@ BeesRoots::crawl_roots()
break;
}
case SCAN_MODE_ONE: {
// Scan each subvol one extent at a time (good for continuous forward progress)
size_t batch_count = 0;
@ -303,6 +306,29 @@ BeesRoots::crawl_roots()
break;
}
case SCAN_MODE_TWO: {
// Scan oldest crawl first (requires maximum amount of temporary space)
vector<shared_ptr<BeesCrawl>> crawl_vector;
for (auto i : crawl_map_copy) {
crawl_vector.push_back(i.second);
}
sort(crawl_vector.begin(), crawl_vector.end(), [&](const shared_ptr<BeesCrawl> &a, const shared_ptr<BeesCrawl> &b) -> bool {
auto a_state = a->get_state();
auto b_state = b->get_state();
return tie(a_state.m_started, a_state.m_root) < tie(b_state.m_started, b_state.m_root);
});
size_t batch_count = 0;
for (auto i : crawl_vector) {
batch_count += crawl_batch(i);
if (batch_count) {
return;
}
}
break;
}
case SCAN_MODE_COUNT: assert(false); break;
}

1
src/bees.h
View File

@ -564,6 +564,7 @@ public:
enum ScanMode {
SCAN_MODE_ZERO,
SCAN_MODE_ONE,
SCAN_MODE_TWO,
SCAN_MODE_COUNT, // must be last
};