On 7.10.19 г. 23:17 ч., Dennis Zhou wrote:
> Hello,


> With async discard, we try to emphasize discarding larger regions
> and reusing the lba (implicit discard). The first is done by using the
> free space cache to maintain discard state and thus allows us to get
> coalescing for fairly cheap. A background workqueue is used to scan over
> an LRU kept list of the block groups. It then uses filters to determine
> what to discard next hence giving priority to larger discards. While
> reusing an lba isn't explicitly attempted, it happens implicitly via
> find_free_extent() which if it happens to find a dirty extent, will
> grant us reuse of the lba. Additionally, async discarding skips metadata

By 'dirty' I assume you mean not-discarded-yet-but-free extent?

> block groups as these should see a fairly high turnover as btrfs is a
> self-packing filesystem being stingy with allocating new block groups
> until necessary.
> Preliminary results seem promising as when a lot of freeing is going on,
> the discarding is delayed allowing for reuse which translates to less
> discarding (in addition to the slower discarding). This has shown a
> reduction in p90 and p99 read latencies on a test on our webservers.
> I am currently working on tuning the rate at which it discards in the
> background. I am doing this by evaluating other workloads and drives.
> The iops and bps rate limits are fairly aggressive right now as my
> basic survey of a few drives noted that the trim command itself is a
> significant part of the overhead. So optimizing for larger trims is the
> right thing to do.

Do you intend on sharing performance results alongside the workloads
used to obtain them? Since this is a performance improvement patch in
its core that is of prime importance!


> Thanks,
> Dennis

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