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Re: [dm-devel] dm-writeboost testing

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)) {

                        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)) {
                                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 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.

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)) {
                                migrate_mb(cache, seg, mb, dirty_bits, true);


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