On Sat, Nov 25, 2023 at 4:19 AM Tony Liu <tonyliu0592@xxxxxxxxxxx> wrote:
>
> Hi,
>
> The context is RBD on bluestore. I did check extent on Wiki.
> I see "extent" when talking about snapshot and export/import.
> For example, when create a snapshot, we mark extents. When
> there is write to marked extents, we will make a copy.
> I also know that user data on block device maps to objects.
Hi Tony,
An extent as simply a byte range, often defined by an offset where it
starts and a length (denoted as offset~length).
> How "extent" and "object" are related?
In the context of RBD there are several types of extents, but the two
main ones are image extents and object extents. An image extent is at
an offset into the image, while an object extent is at an offset into
a particular object. Any range in the image can be referred to using
either type of extent. For example, assuming default striping:
image extent 9437184~4096
and
object 2 extent 1048576~4096
refer to the same 4096-byte block (objects are 0-indexed). Similarly:
image extent 12578816~8192
and
object 2 extent 4190208~4096
object 3 extent 0~4096
refer to the same 8192-byte block, but in this case the image extent
corresponds to two object extents.
Thanks,
Ilya
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