Hi, Mikulas,
Thank you for reporting.
I am really happy to see this report.
First, I respond to the performance problem.
I will make time later for investigating the rest and answer.
Some deadlock issues are difficult to solve in short time.
> I tested dm-writeboost with disk as backing device and ramdisk as cache
> device. When I run mkfs.ext4 on the dm-writeboost device, it writes data
> to the cache on the first time. However, on next mkfs.ext4 invocations,
> dm-writeboost writes data to the disk, not to the cache.
>
> mkfs.ext4 on raw disk: 1.5s
> mkfs.ext4 on dm-cache using raw disk and ramdisk:
> 1st time - 0.15s
> next time - 0.12s
> mkfs.ext4 on dm-writeboost using raw disk and ramdisk:
> 1st time - 0.11s
> next time - 1.71s, 1.31s, 0.91s, 0.86s, 0.82s
>
> - there seems to be some error in logic in dm-writeboost that makes it not
> cache writes if these writes are already placed in the cache. In
> real-world scenarios where the same piece of disk is overwritten over and
> over again (for example journal), this could cause performance problems.
>
> dm-cache doesn't have this problem, if you overwrite the same piece of
> data again and again, it goes to the cache device.
It is not a bug but should/can be optimized.
Below is the cache hit path for writes.
writeboost performs very poorly when a partial write hits
which then turns `needs_cleanup_perv_cache` to true.
Partial write hits is believed to be unlikely so
I decided to give up this path to make other likely-paths optimized.
I think this is just a tradeoff issue of what to be optimized the most.
if (found) {
if (unlikely(on_buffer)) {
mutex_unlock(&cache->io_lock);
update_mb_idx = mb->idx;
goto write_on_buffer;
} else {
u8 dirty_bits = atomic_read_mb_dirtiness(seg, mb);
/*
* First clean up the previous cache
* and migrate the cache if needed.
*/
bool needs_cleanup_prev_cache =
!bio_fullsize || !(dirty_bits == 255);
if (unlikely(needs_cleanup_prev_cache)) {
wait_for_completion(&seg->flush_done);
migrate_mb(cache, seg, mb, dirty_bits, true);
}
I checked that the mkfs.ext4 writes only in 4KB size
so it is not gonna turn the boolean value true for going into the slowpath.
Problem:
Problem is that
it chooses the slowpath even though the bio is full-sized overwrite
in the test.
The reason is that the dirty bits is sometimes seen as 0
and the suspect is the migration daemon.
I guess you created the writeboost device with the default configuration.
In that case migration daemon always works and
some metadata is cleaned up in background.
If you turns both enable_migration_modulator and allow_migrate to 0
before beginning the test
to stop migration at all
it never goes into the slowpath with the test.
Solution:
Changing the code to
avoid going into the slowpath when the dirty bits is zero
will solve this problem.
And done. Please pull the latest one from the repo.
--- a/Driver/dm-writeboost-target.c
+++ b/Driver/dm-writeboost-target.c
@@ -688,6 +688,14 @@ static int writeboost_map(struct dm_target *ti, struct bio *bio
bool needs_cleanup_prev_cache =
!bio_fullsize || !(dirty_bits == 255);
+ /*
+ * Migration works in background
+ * and may have cleaned up the metablock.
+ * If the metablock is clean we need not to migrate.
+ */
+ if (!dirty_bits)
+ needs_cleanup_prev_cache = false;
+
if (unlikely(needs_cleanup_prev_cache)) {
wait_for_completion(&seg->flush_done);
migrate_mb(cache, seg, mb, dirty_bits, true);
Thanks,
Akira
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