Re: [PATCH v9 38/41] btrfs: extend zoned allocator to use dedicated tree-log block group

From: Naohiro Aota <naohiro.aota@wdc.com>
To: Josef Bacik <josef@toxicpanda.com>
Cc: linux-btrfs@vger.kernel.org, dsterba@suse.com, hare@suse.com,
	linux-fsdevel@vger.kernel.org,
	Johannes Thumshirn <johannes.thumshirn@wdc.com>
Subject: Re: [PATCH v9 38/41] btrfs: extend zoned allocator to use dedicated tree-log block group
Date: Tue, 10 Nov 2020 15:37:41 +0900	[thread overview]
Message-ID: <20201110063741.reca4c72vglfylvw@naota.dhcp.fujisawa.hgst.com> (raw)
In-Reply-To: <eb8f83f2-fb59-2b65-66e2-18cd0ecd1e02@toxicpanda.com>

On Tue, Nov 03, 2020 at 03:47:33PM -0500, Josef Bacik wrote:
>On 10/30/20 9:51 AM, Naohiro Aota wrote:
>>This is the 1/3 patch to enable tree log on ZONED mode.
>>
>>The tree-log feature does not work on ZONED mode as is. Blocks for a
>>tree-log tree are allocated mixed with other metadata blocks, and btrfs
>>writes and syncs the tree-log blocks to devices at the time of fsync(),
>>which is different timing from a global transaction commit. As a result,
>>both writing tree-log blocks and writing other metadata blocks become
>>non-sequential writes that ZONED mode must avoid.
>>
>>We can introduce a dedicated block group for tree-log blocks so that
>>tree-log blocks and other metadata blocks can be separated write streams.
>>As a result, each write stream can now be written to devices separately.
>>"fs_info->treelog_bg" tracks the dedicated block group and btrfs assign
>>"treelog_bg" on-demand on tree-log block allocation time.
>>
>>This commit extends the zoned block allocator to use the block group.
>>
>>Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>>Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>
>If you're going to remove an entire block group from being allowed to 
>be used for metadata you are going to need to account for it in the 
>space_info, otherwise we're going to end up with nasty ENOSPC corners 
>here.

Indeed. I'll add a dedicated space_info for treelog or, at least, separate
the block group from other metadata space_info. But, I'll address this
later in v11.

>
>But this begs the question, do we want the tree log for zoned?  We 
>could just commit the transaction and call it good enough.  We lose 
>performance, but zoned isn't necessarily about performance.

We have a large performance drop without tree-log (-o notreelog). Here is a
dbench (32 clients) result on SMR HDD.

With treelog:    153.509  MB/s	
Without treelog:  21.9651 MB/s

So, there is 85% drop of the throughput. I think this degradation is too large.

>
>If we do then at a minimum we're going to need to remove this block 
>group from the space info counters for metadata.  Thanks,
>
>Josef