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readahead: ignore large and unproductive readahead requests

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Sometimes there are absurdly large readahead requests (e.g. 32G),
which tie up a thread holding the readahead lock for a long time (not
to mention the IO the reading hammers the rest of the system with).

These are likely an artifact of the legacy ExtentWalker code interacting
with concurrent filesystem changes.

The maximum btrfs extent size is 128M, so cap the length of readahead
requests at that size.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell
2025-07-21 21:01:00 -04:00
parent fb63bd7e06
commit e87f6e9649
1 changed files with 1 additions and 1 deletions
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src/bees.cc
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@@ -253,7 +253,7 @@ bees_readahead_nolock(int const fd, const off_t offset, const size_t size)
// The btrfs kernel code does readahead with lower ioprio
// and might discard the readahead request entirely.
BEESNOTE("emulating readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
auto working_size = size;
auto working_size = min(size, uint64_t(128 * 1024 * 1024));
auto working_offset = offset;
while (working_size) {
// don't care about multithreaded writes to this buffer--it is garbage anyway