Re: [dm-devel] dm-writeboost testing

[Akira Hayakawa]

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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