Re: [PATCH v9 01/11] btrfs: add raid stripe tree definitions
From: Johannes Thumshirn
Date: Fri Sep 15 2023 - 05:59:20 EST
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.