Re: [PATCH v7 00/19] xfs: Delayed Ready Attrs

Allison Collins
2020-02-23T16:02:36Z

On 2/23/20 12:55 AM, Amir Goldstein wrote:
> On Sun, Feb 23, 2020 at 4:06 AM Allison Collins
> <allison.henderson@oracle.com> wrote:
>>
>> Hi all,
>>
>> This set is a subset of a larger series for delayed attributes. Which is
>> a subset of an even larger series, parent pointers. Delayed attributes
>> allow attribute operations (set and remove) to be logged and committed
>> in the same way that other delayed operations do. This allows more
>> complex operations (like parent pointers) to be broken up into multiple
>> smaller transactions. To do this, the existing attr operations must be
>> modified to operate as either a delayed operation or a inline operation
>> since older filesystems will not be able to use the new log entries.
>
> High level question, before I dive into the series:
>
> Which other "delayed operations" already exist?
> I think delayed operations were added by Darrick to handle the growth of
> translation size due to reflink. Right? So I assume the existing delayed
> operations deal with block accounting.
Gosh, quite a few I think, but I'm not solid on what they all do. If we
take a peek at XFS_LI_TYPE_DESC, theres an identifier for each type, to
give you an idea. A lot of them do look like they are part of reflink
operations though.

> When speaking of parent pointers, without having looked into the details yet,
> it seem the delayed operations we would want to log are operations that deal
> with namespace changes, i.e.: link,unlink,rename.
> The information needed to be logged for these ops is minimal.
> Why do we need a general infrastructure for delayed attr operations?
>
> Thanks,
> Amir.
>
Great question, this one goes back a ways. I believe the train of logic
we had is that because parent pointers also include the filename of the
parent, its possible we can end up with really big attributes. Which
may run into a lot of block map/unmap activity for name space changes.
We didnt want to end up with overly large transactions in the log, so we
wanted to break them up by returning -EAGAIN where ever the transactions
used to be rolled. I'm pretty sure that covers a quick high level
history of where we are now? Did that answer your question?

Allison