linux-kernel - Re: [PATCH v9 01/11] btrfs: add raid stripe tree definitions

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Date:   Fri, 15 Sep 2023 09:55:36 +0000
From:   Johannes Thumshirn <Johannes.Thumshirn@....com>
To:     Qu Wenruo <wqu@...e.com>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>
CC:     Christoph Hellwig <hch@....de>,
        Naohiro Aota <Naohiro.Aota@....com>,
        Damien Le Moal <dlemoal@...nel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 01/11] btrfs: add raid stripe tree definitions

On 15.09.23 02:27, Qu Wenruo wrote:
>>>    /*
>>>     * Records the overall state of the qgroups.
>>>     * There's only one instance of this key present,
>>> @@ -719,6 +724,32 @@ struct btrfs_free_space_header {
>>>        __le64 num_bitmaps;
>>>    } __attribute__ ((__packed__));
>>> +struct btrfs_raid_stride {
>>> +    /* The btrfs device-id this raid extent lives on */
>>> +    __le64 devid;
>>> +    /* The physical location on disk */
>>> +    __le64 physical;
>>> +    /* The length of stride on this disk */
>>> +    __le64 length;
> 
> Forgot to mention, for btrfs_stripe_extent structure, its key is
> (PHYSICAL, RAID_STRIPE_KEY, LENGTH) right?
> 
> So is the length in the btrfs_raid_stride duplicated and we can save 8
> bytes?

Nope. The length in the key is the stripe length. The length in the 
stride is the stride length.

Here's an example for why this is needed:

wrote 32768/32768 bytes at offset 0
XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote 131072/131072 bytes at offset 0
XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
wrote 8192/8192 bytes at offset 65536
XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

[snip]

         item 0 key (XXXXXX RAID_STRIPE_KEY 32768) itemoff XXXXX itemsize 32
                         encoding: RAID0
                         stripe 0 devid 1 physical XXXXXXXXX length 32768
         item 1 key (XXXXXX RAID_STRIPE_KEY 131072) itemoff XXXXX 
itemsize 80
                         encoding: RAID0
                         stripe 0 devid 1 physical XXXXXXXXX length 32768
                         stripe 1 devid 2 physical XXXXXXXXX length 65536
                         stripe 2 devid 1 physical XXXXXXXXX length 32768
         item 2 key (XXXXXX RAID_STRIPE_KEY 8192) itemoff XXXXX itemsize 32
                         encoding: RAID0
                         stripe 0 devid 1 physical XXXXXXXXX length 8192

Without the length in the stride, we don't know when to select the next 
stride in item 1 above.